Semiconductor-oxides nanotubes-based composite particles useful for dye-removal and process thereof

ABSTRACT

Semiconductor-oxide nanotubes-based composite particles are useful for dye-removal. A method involves an ion-exchange mechanism operating under a dark-condition in an aqueous solution, for the processing of products consisting of the nanotubes of semiconductor-oxides deposited on, anchored to or attached to the surface of fly ash particles and metal-oxide (magnetic and non-magnetic) nanoparticles. The resulting micro-nano and nano-nano integrated composite particles can be used in the removal of an organic synthetic-dye from an aqueous solution and industry effluent via a surface-adsorption process, involving ion-exchange and electrostatic-attraction mechanisms. The composite particles can be recycled for the next cycle of dye-adsorption by decomposing the previously adsorbed dye on their surfaces via the use of either noble-metal-deposited or magnetically separable magnetic photocatalysts and exposure to ultraviolet (UV) or solar-radiation.

TECHNICAL FIELD OF THE INVENTION

The present invention primarily relates to semiconductor oxide nanotubesbased composite particles useful for dye removal and process thereof.Particularly, the present invention relates to Semiconductor OxideNanotubes-Flyash and Semiconductor Oxide Nanotubes-Metal Oxide CompositeParticles, their Preparation, method for removal of dye using thesecomposite particles. More particularly, the present invention relates toa method for Recycling of these composite particles in the Dye-RemovalApplication.

BACKGROUND AND PRIOR ART OF THE INVENTION

Organic synthetic-dyes are extensively used in various industries suchas the textile, leather tanning, paper production, food technology,agricultural research, light-harvesting arrays, photo-electrochemicalcells, and hair-coloring. Due to their large-scale production, extensiveuse, and subsequent discharge of colored waste-waters, the toxic andnon-biodegradable organic synthetic-dyes cause considerableenvironmental pollution and health-risk factors. Moreover, they alsoaffect the sunlight penetration and oxygen solubility in thewater-bodies, which in turn affect the under-water photosynthesisactivity and life-sustainability. Moreover, due to their strong coloreven at lower concentrations, the organic synthetic-dyes generateserious aesthetic issues in the waste-water disposal. On the other hand,the toxic soluble heavy metal-cations cause serious problems to theecosystem due to the serious health problems as a result of theiraccumulation in living tissues through the food-chain. Therefore, theremoval of both highly stable organic synthetic-dyes and heavymetal-cations from the aqueous solutions and industry effluents is of aprime importance. Reference may be made to (V. K. Gupta, Suhas,“Application of Low-Cost Adsorbents for Dye Removal—A Review”, Journalof Environmental Management 2009, 90, 2313-2342) wherein, theinvestigations conducted in the past using the different adsorbents suchas orange peel, rice husk, coconut shell, carbon black, zeolites, carbonnanotubes, and flyash have been summarized. Typically, flyash (solid andhollow, also known as cenospheres) which is a waste by-product ofthermal power plants, comprising the mixture of different metal-oxidessuch as silica (SiO₂, 50-85 wt. %), alumina (Al₂O₃, 5-20 wt. %), ironoxide (Fe₂O₃, 5-15 wt. %), and trace amount of oxides of other elementssuch as calcium, titanium, magnesium, and toxic heavy metals such asarsenic, lead, and cobalt, has been environmentally hazardous and posemajor disposal and recycling problems worldwide. Reference may be madeto (H. Liu, “Method to Produce Durable Non-Vitrified Fly Ash Bricks andBlocks”, U.S. Pat. No. 7,998,268; B. R. Reddy, K. M. Ravi, “Methods ofFormulating a Cement Composition”, U.S. Pat. No. 7,913,757; R. L. Hill,C. R. Jolicoeur, R. Carmel, M. Page, I. Spiratos, T. C. To, “SacrificialAgents for Fly Ash Concrete”, U.S. Pat. No. 7,901,505) wherein, flyashhas been traditionally used for landfill, manufacturing constructionalmaterials such as cement, concrete, and bricks. Reference may be made to(K. Vasanth Kumar, V. Ramamurthi, S. Srinivasan, “Modeling the MechanismInvolved During the Sorption of Methylene Blue onto Flyash”, Journal ofColloid and Interface Science 2005, 284, 14-21; M. Matheswaran, T.Karunanithi, “Adsorption of Chrysiodine R by using Fly Ash in BatchProcess”; Journal of Hazardous Materials 2007, 145, 154-161; D. Mohan,K. P. Singh, G. Singh, K. Kumar, “Removal of Dyes from Wastewater UsingFlyash a Low-Cost Adsorbent”, Industrial and Engineering ChemistryResearch 2002, 41, 3688-3695; S. Kara, C. Aydiner, E. Demirbas, M.Kobya, N. Dizge, “Modeling the Effects of Adsorbent Dose and ParticleSize on the Adsorption of Reactive Textile Dyes by Fly Ash”,Desalination 2007, 212, 282-293) wherein, new industrial applications offlyash such as conductive/non-conductive filler in polymers, binder foragglomerating reactive mine tailings, manufacturing zeolites andmicrofiltration membranes, and adsorption of oil from aqueous solutionshave been demonstrated. Flyash has also been utilized for the adsorptionof different organic synthetic-dyes including remazol red RB 133,remazol blue, rifacion yellow HED, chrysoidine R, crystal violet,Rhodamine B, C.I. reactive black, 2-picoline, and acid red (AR1) fromthe aqueous solutions. Reference may be made to (M. Visa, C. Bogatu, A.Duta, “Simultaneous Adsorption of Dyes and Heavy Metals fromMulticomponent Solutions using Fly Ash”, Applied Surface Science 2010,256, 5486-5491; S. Wang, M. Soudi, L. Li, Z. H. Zhu, “Coal AshConversion into Effective Adsorbents for Removal of Metals and Dyes fromWastewater”, Journal of Hazardous Materials 2006, B133, 243-252; K.Ojha, N. C. Pradhan, A. N. Samanta, “Zeolite from Fly Ash: Synthesis andCharacterization”, Bulletin of Materials Science 2004, 27, 555-563)wherein, flyash has been utilized for the adsorption of heavymetal-cations such as Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺,Ti⁴⁺, and others from aqueous solutions. The major advantage of usingflyash for these applications is that it can be separated from thetreated aqueous solutions via gravity settling. However, the majordrawbacks of prior art-1 are as follows.

-   -   (1) Flyash has very low specific surface-area, and as a result,        exhibits very low capacity for adsorbing organic synthetic-dyes        and heavy metal-cations on its surface.    -   (2) Novel techniques to increase the specific surface-area of        flyash, without affecting its typical spherical morphology and        yet increasing its capacity for adsorbing organic synthetic-dyes        and heavy metal-cations, are unknown.    -   (3) Adsorption of organic synthetic-dyes and heavy metal-cations        on the surface of flyash generates large amount of sludge which        creates further handling, disposal, and recycling issues which        are at present not addressed satisfactorily.    -   (4) Novel value-added products based on the innovative        surface-modifications of flyash for the potential applications,        typically the removal of organic synthetic-dyes from the aqueous        solutions, are currently lacking.    -   (5) Novel approaches for recycling the flyash as a catalyst in        the dye-removal application, by decomposing the previously        adsorbed-dye from its surface, are currently lacking.

Hence, it is vital to develop innovative approaches to enhance thespecific surface-area of flyash to increase its capacity for theadsorption of organic synthetic-dyes and heavy metal-cations. Innovativeapproaches are also needed to be developed to decompose the previouslyadsorbed-dye from the surface of flyash to recycle it for thenext-cycles of dye-adsorption as a catalyst Reference may be made to (S.Shukla, S. Seal, J. Akesson, R. Oder, R. Carter, K. Scammon, “Study ofMechanism of Electroless Copper Coating of Flyash Cenosphere Particles”,Applied Surface Science 2001, 181, 35-50; S. Shukla, S. Seal, Z.Rahaman, K. Scammon, “Electroless Copper Coating of Cenospheres usingSilver Nitrate Activator”, Materials Letters 2002, 57, 151-156; S.Shukla, K. G. K. Warder, K. V. Baiju, T. Shijitha, “NovelSurface-Modifications for Flyash and Industrial Applications Thereof”,U.S. patent application Ser. No. 13/612,363 (Filed on 12 Sep. 2012), PCTApplication No. PCT/IN2010/000735 (Filed on 11 Nov. 2010)) wherein, asfar as flyash particles with the surface-adsorbed heavy metal-cationsare concerned, the Sn²⁺ cations adsorbed on the surface of flyashparticles have been reported to be useful as sensitizer in anelectroless metal (Cu/Ag)-coating of flyash particles using the Sn—Pdcatalyst system. However, the major drawbacks of prior art-2 are asfollows.

-   -   (6) Flyash particles with the surface-adsorbed Sn²⁺ cations do        not find other novel potential industrial applications.    -   (7) Flyash particles with the surface-adsorbed metal-cations,        other than Sn²⁺, are not suitable for the electroless        metal-coating application.    -   (8) Flyash particles with the surface-adsorbed metal-cations,        other than Sn²⁺, have not been utilized for other potential        applications.

Hence, the new potential industrial applications are required to beinvented for improving the handling, disposal, and recycling issues offlyash with the surface-adsorbed heavy metal-cations. Reference may bemade to (T. Kasuga, H. Masayoshi, “Crystalline Titania and Process forProducing the Same”, U.S. Pat. No. 6,027,775; T. Kasuga, H. Masayoshi,“Crystalline Titania having Nanotube Crystal Shape and Process forProducing the Same”; U.S. Pat. No. 6,537,517; N. Harsha, K. R. Ranya, S.Shukla, S. Biju, M. L. P. Reddy, K. G. K. Warder; “Effect of Silver andPalladium on Dye-Removal Characteristics of Anatase-Titania Nanotubes”,Journal of Nanoscience and Nanotechnology 2011, 11, 2440-2449; N.Harsha, K. R. Ranya, K. B. Babitha, S. Shukla, S. Biju, M. L. P. Reddy,K. G. K. Warder, Hydrothermal Processing of HydrogenTitanate/Anatase-Titania Nanotubes and Their Application as StrongDye-Adsorbents”, Journal of Nanoscience and Nanotechnology 2011, 11,1175-1187; P. Hareesh, K. B. Babitha, S. Shukla, “Processing Fly AshStabilized Hydrogen Titanate Nano-Sheets for Industrial Dye-RemovalApplication”, Journal of Hazardous Materials 2012, 229-230, 177-182)wherein, the removal of heavy metal-cations and organic synthetic-dyesfrom the aqueous solutions via the surface-adsorption process, involvingthe ion-exchange and electrostatic-attraction mechanisms operating inthe dark-condition, using the hydrothermally processed nanotubes ofsemiconductor-oxides such as the hydrogen titanate (H₂Ti₃O₇, HTN) andanatase-titania (TiO₂, ATN) have been demonstrated. The HTN and ATNpossess very high specific surface-area typically about 100-200 timesthat of as-received flyash particles. Hence, the adsorption-capacity ofHTN and ATN for adsorbing organic synthetic-dyes and heavy metal-cationsis extremely large. However, the major drawbacks of prior art-3 are asfollows.

-   -   (9) HTN and ATN cannot be separated quickly from the treated        aqueous solution via gravity settling.    -   (10) HTN and ATN are non-magnetic; hence, they cannot be        separated from the treated aqueous solution using an external        magnetic field.

In view of the prior arts 1-3 and their limitations, it appears thatthere is a need for the development of novel composite materials whichwould exhibit higher capacities for surface-adsorbing the organicsynthetic-dyes and heavy metal-cations and can be separated quickly fromthe treated aqueous solution via gravity-settling or using an externalmagnetic field. Flyash particles have lower dye-adsorption capacity butcan be separated from the treated aqueous solution via the gravitysettling. On the other hand, the nanotubes of semiconductor-oxides havehigher dye-adsorption capacity but cannot be separated from the treatedaqueous solution via the gravity settling. Hence, this suggests that amicro-nano composite material consisting of the nanotubes ofsemiconductor-oxides, such as the hydrothermally processed HTN and ATN,deposited on the surface of flyash particles can serve the purpose.Reference may be made to (S. Shukla, K. G. K. Waffler, M. R. Varma, M.T. Lajina, N. Harsha, C. P. Reshmi, “Magnetic Dye-Adsorbent Catalyst”,U.S. patent application Ser. No. 13/521,641 (Filed on 11 Jul. 2012), PCTApplication No. PCT/IN2010/000198 (Filed on 29 Mar. 2010); L. Thazhe, A.Shereef, S. Shukla, R. Pattelath, M. R. Varma, K. G. Suresh, K. Patil,K. G. K. Warrier, “Magnetic Dye-Adsorbent Catalyst: Processing,Characterization, and Application”, Journal of American Ceramic Society2010, 93(11), 3642-3650) wherein, the magnetic dye-adsorbent catalyst,consisting of the “core-shell” nanocomposite particles with the core ofa magnetic ceramic particle and the shell of nanotubes ofsemiconductor-oxide, has been developed via the hydrothermal treatmentof magnetic photocatalyst (processed via the Stober and sol-gel methods)followed by typical washing-cycles, to facilitate the quick settling ofHTN and ATN using an external magnetic field. Reference may be made to(C. C. Sheng, L. T. Gui, C. X. Hua, L. L. Wu, L. Q. Cheng, XQing, N. Z.Wu, “Preparation and Magnetic Property of Multi-Walled CarbonNanotube/α-Fe ₂ O ₃ Composites”, Transactions of Nonferrous MetalsSociety of China, 2009, 19, 1567-1571; E. Santala, M. Kemell, M.Leskelä, M. Ritala, “The Preparation of Reusable Magnetic andPhotocatalytic Composite Nanofibers by Electrospinning and Atomic LayerDeposition”, Nanotechnology 2009, 20, 035602; S. K Mohapatra, S.Banerjee, M. Misra, “Synthesis of Fe ₂ O ₃ /TiO ₂ Nanorod-NanotubeArrays by Filling TiO ₂ Nanotubes with Fe”, Nanotechnology 2008, 19315601, Fei Liu, Yinji Jin, Hanbin Liao, Li Cai, Meiping Tong, YanglongHou, “Facile Self-Assembly Synthesis of Titanate/Fe ₃ O ₄ Nanocompositesfor the Efficient Removal of Pb ²⁺ from Aqueous Systems”, Journal ofPhysical Chemistry A, DOI: 10.1039/c2ta00099g) wherein, the magneticnanocomposites having the morphology other than the “core-shell”morphology, consisting of magnetic nanoceramic particles deposited onthe surface of TiO₂ nanotubes, have also been processed via differenttechniques including the precipitation-calcination,electrospinning-atomic layer deposition, pulsed electrodeposition, andself-assembly process. However, the major drawbacks of prior art-4 areas follows.

-   -   (11) The combination of sol-gel and hydrothermal methods is not        applicable to the flyash particles since SiO₂, a major        constituent of flyash particles, is soluble in a highly alkaline        aqueous solution involved in the hydrothermal treatment.    -   (12) Innovative techniques for depositing the nanotubes of        semiconductor-oxides on the surface of flyash are not known.    -   (13) Other techniques including the precipitation-calcination,        electrospinning-atomic layer deposition, and pulsed        electrodeposition are not suitable for depositing the nanotubes        of semiconductor-oxides on the surface of flyash.    -   (14) The self-assembly process produces the magnetic        nanocomposite with the magnetic nanoparticles uniformly        dispersed on the surface of semiconductor-oxides nanotubes which        reduces the potential sites on the surface of nanotubes required        for the adsorption of dye molecules and the metal-cations from        the aqueous solutions. Moreover, the self-assembly method also        requires the use of an acid for obtaining the said morphology.    -   (15) Innovative techniques for attaching or anchoring the HTN or        ATN to the surface of magnetic nanoparticles, typically at their        short-edges (tube-openings), are not currently available. As a        result, the said product cannot be synthesized using any of the        existing processes.

As a consequence, there is an urgent need to develop novel methods fordepositing the nanotubes of semiconductor-oxides on the surface offlyash particles. Since the flyash particles are non-magnetic, theycannot be separated from an aqueous solution using an external magneticfield. Hence, it is also essential to demonstrate the deposition of thenanotubes of semiconductor-oxides on the surface of magnetic metal-oxidenanoparticles (instead of flyash) by attaching or anchoring them to themagnetic particle-surface using the same innovative mechanism which isemployed in the case of flyash particles. Thus, the novel compositematerials consisting of the nanotubes of semiconductor-oxides depositedon the surface of both the non-magnetic flyash and attached to (oranchored to) the surface of magnetic metal-oxide nanoparticles, via aninnovative approach, would provide new ways of efficiently treating theaqueous solutions containing the harmful organic synthetic-dyes andheavy metal-cations. It would also provide new ways for the separationand recycling the flyash, without and with the surface-adsorbedmetal-cations, as value-added products for the dye-removal application.As mentioned above, flyash (without and with the surface-adsorbedmetal-cations), HTN, ATN, and magnetic composites can be used asdye-adsorbents. In order to recycle these dye-adsorbents as catalystsfor the next-cycles of dye-adsorption, it is necessary to remove thepreviously-adsorbed dye from their surfaces. Reference may be made to(Z. Geng, Y. Lin, X. Yu, Q. Shen, L. Ma, Z. Li, N. Pan, X. Wang, “HighlyEfficient Dye Adsorption and Removal: A Functional Hybrid of ReducedGraphene Oxide-Fe ₃ O ₄ Nanoparticles as an Easily RegenerativeAdsorbent”, Journal of Materials Chemistry 2012, 22, 3527-3535; M. Visa,L. Andronic, D. Lucaci, A. Duta, “Concurrent Dyes Adsorption andPhoto-Degradation on Fly Ash Based Substrates”, Adsorption 2011, 17,101-108; J.-T. Li, B. Bai, Y-L. Song, “Degradation of Acid Orange 3 inAqueous Solution by Combination of Fly Ash/H ₂ O ₂ and UltrasoundIrradiation”, Indian Journal of Chemical Technology 2010, 17, 198-203)wherein, annealing under the moderate conditions (at 400° C. for 1 h)for removing the previously adsorbed Rhodamine B dye from the surface ofreduced graphene oxide-Fe₃O₄ composite has been reported. The mechanicalmixture of flyash and TiO₂ powders has been employed for thedecomposition of organic synthetic-dye on the surface of flyash underthe ultraviolet (UV)-radiation exposure. The recycling of flyash via thesimultaneous dye-adsorption on its surface and dye-degradation using thecombination of H₂O₂ and ultrasound-irradiation (Fenton-like reaction)has been reported. Reference may be made to (S. Shukla, K. G. K.Warrier, K. B. Babitha, “Methods for Decomposition of OrganicSynthetic-Dyes using Semiconductor-Oxides Nanotubes via Dark-Catalysis”,PCT Application No. PCT/IN2013/000319 (Filed on 17 May 2013), IndianPatent Application 2555DEL2012 (Filed on 17 Aug. 2012)) wherein, thecombination of hydrothermally processed HTN or ATN and H₂O₂ has beenused to degrade the previously adsorbed organic synthetic-dye in anaqueous solution, typically in the dark-condition, without the use ofexternal-irradiation and external power-source. In such case, thedye-decomposition is achieved through the generation and attack of boththe free hydroxyl-radicals (OH⁻) and superoxide-ions (O₂ ⁻) which aregenerated by the HTN and ATN in the presence of H₂O₂. Reference may bemade to (M. S. Yalfani, S. Contreras, F. Medina, J. Sueiras, “DirectGeneration of Hydrogen Peroxide from Formic Acid and O ₂ usingHeterogeneous Pd/α-Al ₂ O ₃ Catalysts”, Chemical Communications, 2008,3885-3887) wherein, the Pd-deposited on alumina (Al₂O₃) substrate hasbeen utilized to generate H₂O₂ in-situ using the formic acid (HCOOH) anddissolved oxygen (O₂) which can be utilized for the decomposition oforganic synthetic-dyes in an aqueous solution via the Fenton/Fenton-likereactions. However, the major drawbacks of prior-art 5 are as follows.

-   -   (16) Innovative techniques for removing the previously        adsorbed-dye from the surface of various adsorbents mentioned        above, for recycling them as catalysts, are unknown.    -   (17) The annealing treatment, conducted even under the moderate        conditions, can destroy the nanotube-morphology and the phase        structure;    -   (18) Flyash and TiO₂ cannot be separated, after the        dye-decomposition, from their mechanical mixture since both are        non-magnetic; hence, the recycling of flyash is not possible        from the mechanical mixture of flyash and TiO₂ powders for        reusing the former separately for the dye-adsorption process        conducted in the dark-condition.    -   (19) The use of a magnetic photocatalyst, consisting of the        “core-shell” nanocomposite particles with the core of a magnetic        ceramic particle, an intermediate insulating layer of silica        (SiO₂) or an organic polymer, and the shell of nanocrystalline        particles of semiconductor-oxide photocatalyst such as        anatase-TiO₂, has never been reported for the recycling of        dye-adsorbents typically the flyash without and with the        surface-modifications.    -   (20) The adsorbents such as the HTN and ATN cannot be recycled        via the mechanically mixing and UV-exposure method involving the        use of TiO₂ photocatalyst since the nanocrystalline TiO₂        particles have a tendency to get attached to the HTN and ATN via        an ion-exchange mechanism (which has been disclosed here) which        reduces the total number of potential-sites available for the        dye-adsorption for a given amount of dye-adsorbent. The latter        issue becomes severe with the increasing number of        dye-adsorption cycles.    -   (21) The recycling of flyash particles using the combination of        H₂O₂ (Fenton-like reactions) and ultrasound-irradiation is a        costlier process.    -   (22) The efficient methods to recycle the flyash, in the        dye-removal application, without the use of external-power        source are not known.

Hence, it is essential to develop simpler, easier, cost-effective,efficient, and innovative processes to remove or decompose thepreviously adsorbed-dye from the surface of dye-adsorbents, typicallythe flyash without and with the surface-modifications, to make theirrecycling possible in the dye-removal application.

OBJECTIVE OF THE INVENTION

The main objective of the present invention is to providesemiconductor-oxides nanotubes-based composite particles useful fordye-removal and process thereof.

Another objective of the present invention is to providesemiconductor-oxide nanotubes-based composite particles typicallyrelated to the potential application concerning the removal of organicsynthetic-dyes from the aqueous solutions via the surface-adsorptionprocess, involving the ion-exchange and electrostatic-attractionmechanisms, operating in the dark-condition.

Still another objective of the present invention is to provide theprocesses for the preparation of semiconductor-oxide nanotubes-flyashand semiconductor-oxide nanotubes-metal oxide composite particles.

Yet another objective of the present invention is to provide innovativeproducts, and processes for the recycling of flyash, with thesurface-adsorbed metal-cations, as a value-added waste by-product.

Yet another objective of the present invention is to provide a methodinvolving both the surface-sensitization of flyash particles (byadsorbing the cations (either metal-ions (M^(n+)) or protons (H⁺)) andan ion-exchange mechanism operating under the dark-condition in anaqueous solution for the processing of novel semiconductor-oxidenanotubes-flyash composite particles.

Yet another objective of the present invention is to provide the optimumparameters for the processing of semiconductor-oxide nanotubes-flyashcomposite particles via a method involving both thesurface-sensitization of flyash particles and an ion-exchange mechanismoperating under the dark-condition in an aqueous solution.

Yet another objective of the present invention is to provide thedoped-nanotubes of semiconductor-oxides to increase the dye-adsorptioncapacity of semiconductor-oxide nanotubes-flyash composite particles forthe dye-removal application.

Yet another objective of the present invention is to provide a processfor the preparation of semiconductor-oxide nanotubes-metal oxidenanocomposite particles which does not involve the surface-sensitizationstep.

Yet another objective of the present invention is to provide a methodfor decomposing the previously adsorbed-dye on the surface ofas-received flyash particles or the semiconductor-oxide nanotubes-flyashcomposite particles (non-magnetic), by using the combination of amagnetic photocatalyst and an exposure to the UV or solar-radiation torecycle the as-received flyash particles or the semiconductor-oxidenanotubes-flyash composite particles as catalyst for the repeated cyclesof dye-adsorption conducted in the dark-condition.

Yet another objective of the present invention is to provide a methodfor decomposing the previously adsorbed-dye on the surface ofsemiconductor-oxide nanotubes-metal oxide magnetic nanocompositeparticles, by using the combination of noble-metal-depositednanocrystalline anatase-TiO₂ photocatalyst (non-magnetic) and anexposure to the UV or solar-radiation, to recycle the magneticnanocomposite as a catalyst for the repeated cycles of dye-adsorptionconducted in the dark-condition.

Yet another objective of the present invention is to provide thesemiconductor-oxide nanotubes-flyash composite particles which aremagnetic in nature.

Yet another objective of the present invention is to provide a processfor the preparation of semiconductor-oxide nanotubes-flyash compositeparticles which are magnetic in nature.

Yet another objective of the present invention is to provide thesemiconductor-oxide nanotubes-flyash composite particles which aremagnetic in nature for the dye-removal application.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a composite particle whichcomprises,

-   -   (i) metal-cations (M^(n+)) or protons-sensitized flyash particle        of diameter in the range of 0.3-100 μm,    -   (ii) nanotubes of semiconductor-oxides deposited on the surface        of metal-cations or protons-sensitized flyash particle.

In one embodiment of the present invention the nanotubes are in therange of 1-15 wt. %, the meal-cations (M^(n+)) or protons in the rangeof 1-80 wt. %, and the balance being the weight of flyash particles.

In an embodiment of the present invention the flyash particle consistsof the mixture of silica (SiO₂, 50-85 wt. %), alumina (Al₂O₃, 5-20 wt.%), iron oxide (Fe₂O₃, 5-15 wt. %), and trace amount of oxides of otherelements selected from calcium, titanium, magnesium, and toxic heavymetals selected from arsenic, lead, and cobalt.

In another embodiment of the present invention the metal-cations(M^(n+)) are selected from the group consisting of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺,Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, and Tr.

Still in another embodiment of the present invention the nanotubes ofsemiconductor-oxides are selected from the group consisting ofhydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or thelepidocrocite-type), anatase-TiO₂ (ATN), and Ag-doped ATN.

Still in another embodiment of the present invention the nanotubes ofsemiconductor-oxides are optionally attached to the magnetic metal-oxidenanoparticles at the short-edges (tube-openings) and/orsurface-deposited with the magnetic metal-oxide nanoparticles which areselected from the group consisting of γ-Fe₂O₃, Fe₃O₄, CoFe₂O₄, NiFe₂O₄,MnFe₂O₄, Co, Fe, and Ni.

Still in another embodiment of the present invention a process for thepreparation of a composite particle, wherein the said process comprisesthe steps of,

-   -   (a) dissolving a metal-salt (1-70 g·l⁻¹) in water having initial        solution-pH-1-2 adjusted using the 0.1-1 M HCI solution under        continuous stirring for 30-300 min at temperature ranging        between 20-30° C., followed by suspending flyash particles (1-25        g·l⁻¹) into it under continuous stirring for 30-300 min at        temperature ranging between 20-30° C. to obtain the suspension        of metal-cations (M^(n+)) sensitized flyash particles;    -   (b) optionally suspending flyash particles (1-25 g·l⁻¹) in        50-250 ml of water having the initial solution-pH-1-2 adjusted        using the 0.1-1 M HCI solution under continuous stirring for        30-300 min at temperature ranging between 20-30° C. to obtain        the suspension of protons-sensitized flyash particles;    -   (c) adding hydrothermally processed nanotubes of        semiconductor-oxides (0.1-5 g·l⁻¹) to the suspension of cations        (M^(n+)) or protons-sensitized flyash particles as obtained in        step (a) or in step (b), followed by sonicating the suspension        for 5-30 min;    -   (d) stirring the suspension obtained in step (c) continuously        for 30-300 min at temperature ranging between 20-30° C. to        obtain the composite of semiconductor-oxide nanotubes and        cations (M^(n+)) or protons-sensitized flyash particles;    -   (e) separating the composite particles as obtained in step (d)        using a centrifuge operated at 2000-4000 rpm or an external        magnetic field, followed by washing the composite particles        using water for 30 min-2 h multiple-times till the pH of        filtrate remains unchanged or neutral, followed by separation        and drying the composite particles in an oven at 70-90° C. for        10-15 h.

Still in another embodiment of the present invention the metal-salt usedin step (a) is selected from the group consisting of chloride, nitrate,sulfate-salts of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁴⁺, andothers.

Still in another embodiment of the present invention a nanocompositeparticle which comprises,

-   -   (i) metal-oxide nanoparticles in the range of 5-70 wt. %;    -   (ii) nanotubes of semiconductor-oxides in the range of 30-95 wt.        % attached to the surface of metal-oxide nanoparticles at the        short-edges (tube-openings).

Still in another embodiment of the present invention the metal-oxidenanoparticles are selected from the group consisting of γ-Fe₂O₃(magnetic), Fe₃O₄ (magnetic), SnO/SnO₂, PbO, ZnO, CuO, MnO, and TiO₂.

Still in another embodiment of the present invention the nanotubes ofsemiconductor-oxides are selected from the group consisting ofhydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or thelepidocrocite-type) and anatase-TiO₂ (ATN).

Still in another embodiment of the present invention a process for thepreparation of a nanocomposite particle, wherein the said processcomprises the steps of,

-   -   (A) Dispersing the 0.5-10 g·l⁻¹ of metal-oxide nanoparticles in        water having the neutral solution-pH (˜6.5-7.5) under continuous        stirring for 5-30 min at temperature in the range of 20-30° C.;    -   (B) adding 0.5-10 g·l⁻¹ of hydrothermally processed nanotubes of        semiconductor-oxides in the suspension obtained in step (A)        under continuous stirring for 5-30 min at temperature in the        range of 20-30° C., followed by sonicating the suspension for        5-30 min, subsequently stirring the suspension continuously for        1-10 h in the dark-condition to obtain the semiconductor-oxide        nanotubes-metal oxide nanocomposite particles;    -   (C) separating the semiconductor-oxide nanotubes-metal oxide        nanocomposite particles using a centrifuge operated at 2000-4000        rpm or an external magnetic field, followed by washing the        nanocomposite particles using water for 30 min-2 h till the pH        of filtrate remains unchanged or neutral, followed by separation        and drying the nanocomposite particles in an oven at 70-90° C.        for 10-15 h.

Still in another embodiment of the present invention The composite andnanocomposite particles are useful for the application involving thedye-removal from an aqueous solution and industry-effluent via thesurface-adsorption mechanism operating in the dark-condition.

Still in another embodiment of the present invention a process for thesurface-cleaning and the recycling of composite/nanocomposite particlesafter the adsorption of an organic synthetic-dye from an aqueoussolution via the surface-adsorption mechanism operating in thedark-condition, comprising the steps of,

-   -   a. suspending 1-30 g·l⁻¹ of composite/nanocomposite particles        having the surface-adsorbed organic synthetic-dye (0.1-3 mg·g⁻¹)        in water under continuous stirring for a period ranging between        5-30 min at temperature ranging between 20-30° C.;    -   b. suspending a photocatalyst (10-60 wt. % of total weight of        suspended solid particles) selected from the group of        nanocrystalline anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic        photocatalyst or the noble-metal-deposited nanocrystalline        anatase-TiO₂ in the suspension obtained in step (a) under        continuous stirring, followed by sonicating the suspension for        5-30 min, subsequently stirring the suspension continuously        under the UV or solar-radiation exposure for 1-10 h;    -   c. centrifuging the solution at 2000-4000 rpm to separate the        composite/nanocomposite particles and photocatalyst together,        followed by washing using water for 30 min-2 h multiple-times        till the pH of filtrate remains unchanged or neutral;    -   d. separating the photocatalyst particles from the        surface-cleaned composite/nanocomposite particles using an        external magnetic field;    -   e. drying both the photocatalyst particles and        composite/nanocomposite particles in an oven at 70-90° C. for        10-15 h for the reuse.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 represents the TEM images of as-received flyash particles (a) andhydrothermally processed HTN (b).

FIG. 2 represents the XRD patterns of as-received flyash particles (a)and hydrothermally processed HTN (b).

FIG. 3 represents the EDX spectra of as-received flyash particles (a)and hydrothermally processed HTN (b).

FIG. 4 represents the high magnification TEM images of as-receivedflyash particle (a) and the micro-nano integrated HTN-flyash compositeparticle (b) showing the surface and interphase regions. In (b), thecomposite particle is processed via an ion-exchange mechanism operatingunder the dark-condition in an aqueous solution using the Sn²⁺ cationsfor the surface-sensitization of as-received flyash particles.

FIG. 5 represents the XRD pattern (a) and EDX spectrum (b) of micro-nanointegrated HTN-flyash composite particles processed via an ion-exchangemechanism, operating under the dark-condition in an aqueous solution,using the Sn²⁺ cations for the surface-sensitization of as-receivedflyash particles. The initial SnCl₂ concentration is 40 g·l⁻¹.

FIG. 6 represents the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time for the differentdissolution time of SnCl₂ which is varied as 30 min (a), 2 h (b), and 4h (c). The initial SnCl₂ concentration is varied as 5 (i), 20 (ii), 40(iii), and 60 g·l⁻¹ (iv). The initial solution-pH during thedye-adsorption measurement is ˜7.5.

FIG. 7 represents the variation in q_(e) as a function of initial SnCl₂concentration obtained for the different dissolution time of SnCl₂ whichis varied as 30 min (a), 2 h (b), and 4 h (c). The initial solution-pHduring the dye-adsorption measurement is ˜7.5. The maximum value ofq_(e) is indicated by an arrow.

FIG. 8 represents the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time obtained for theHTN-flyash composite particles (non-magnetic) processed using thedifferent contact time of SnCl₂ solution with the as-received flyashparticles (a) and that of the hydrothermally processed HTN with theSn²⁺-sensitized flyash particles (b). (i)-(iv) represent the contacttime of 1-4 h at the interval of 1 h. The initial SnCl₂ concentrationand its dissolution time are 20 g·l⁻¹ and 2 h. In (a), the contact timeHTN and Sn²⁺-activated flyash particles is 4 h. In (b), the contact timeSnCl₂ solution with the suspended as-received flyash particles is 4 h.

FIG. 9 represents (a) the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time obtained for thedifferent initial MB dye concentrations which is varied as 15 (i), 30(ii), 60 (iii), and 90 μM (iv) measured at the initial solution-pH of˜7.5. (b) represents calculated variation in q_(e), as a function ofinitial MB dye concentration obtained for the HTN-flyash composite.

FIG. 10 represents the TEM image (a) and XRD pattern (b) ofhydrothermally processed ATN. The precursor used for the hydrothermalprocessing of ATN is the nanocrystalline anatase-TiO₂ particlesprocessed via acetic-acid (catalyst) modified sol-gel method. In (a),the inset shows the corresponding SAED pattern. In (b), A represents theanatase-TiO₂.

FIG. 11 represents the variation in q_(e) as a function of initial MBdye concentration as obtained for the hydrothermally processed Ag-dopedATN (a) and the Ag-doped ATN-flyash composite particles (b) havingdifferent values of Ag/Ti mole-ratio—0 (i), 1 (ii), and 5% (iii).

FIG. 12 represents the TEM image (a), EDX spectrum (b), and XRD pattern(c) obtained using the conventional anatase-TiO₂-coated (cycle-5)SiO₂/γ-Fe₂O₃ magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. %γ-Fe₂O₃, and 49 wt. % SiO₂) processed using the combination ofmodified-Stober and conventional sol-gel methods. A and M represent theanatase-TiO₂ and γ-Fe₂O₃ (maghemite) structures.

FIG. 13 represents the effect of a moderate external magnetic field onthe magnetic separation of anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and49 wt. % SiO₂) from an aqueous solution. (a) and (b) show the digitalphotographs taken after the addition of magnetic photocatalyst to anaqueous solution without and with the presence of a moderate externalmagnetic field (holding time of 5 min). In (b), the arrow at theright-side bottom-corner shows the position of an external magnet.

FIG. 14 represents the variation in the PL intensity associated with theformation of 2-hydroxyterepthalic acid as a function of solar-radiationexposure time as obtained for the as-received flyash particles (a) andanatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalyst(containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂) (b). Theexcitation wavelength is ˜315 nm.

FIG. 15 represents the variation in q_(e), obtained using theas-received flyash particles, as a function of number of dye-adsorptioncycles conducted in the dark-condition. The dye-adsorption cycles 1-4are conducted before and cycle-5 is conducted after applying thesurface-cleaning treatment, conducted under the solar-radiationexposure, to decompose the previously adsorbed-dye from the surface ofas-received flyash particles.

FIG. 16 represents the high magnification TEM images (a,b) and EDXspectrum (c) of micro-nano integrated HTN-flyash composite particles.(a,b) show the interphase regions between the flyash particle andsurface-adsorbed HTN processed via an ion-exchange mechanism, operatingunder the dark-condition in an aqueous solution, using the Fe³⁺ cationsfor the surface-sensitization of as-received flyash particles. TheHTN-flyash composite particles (non-magnetic) contain 20 wt. % Fe and 7wt. % HTN (balance being the weight of flyash particles).

FIG. 17 represents the variation in the normalized concentration ofsurface-adsorbed MB dye as obtained for the as-received flyash particles(i) and HTN-flyash composite particles (non-magnetic) (ii). The latteris processed by using the Fe³⁺ cations for the surface-sensitization ofas-received flyash particles. The initial MB dye concentration and theinitial solution-pH are 30 μM and ˜7.5. The HTN-flyash compositeparticles (non-magnetic) contain 20 wt. % Fe and 7 wt. % HTN (balancebeing the weight of flyash particles).

FIG. 18 represents the TEM image (a) and EDX spectrum (b) obtained usingthe HTN-flyash composite particles processed using the H⁺ ions, insteadof metal-cations (M^(n+)), for the surface-sensitization of as-receivedflyash particles. The HTN-flyash composite particles (non-magnetic)containing ˜2 wt. % H and 9 wt. % HTN (balance being the weight offlyash particles).

FIG. 19 represents the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time as obtained forthe as-received flyash particles (i) and HTN-flyash composite particles(non-magnetic) (ii). The latter is processed using the H⁺ ions, insteadof metal-cations (M^(n+)), for the surface-sensitization of as-receivedflyash particles. The initial MB dye concentration and initialsolution-pH are 15 μM and ˜7.5. The HTN-flyash composite particles(non-magnetic) containing ˜2 wt. % H and 9 wt. % HTN (balance being theweight of flyash particles).

FIG. 20 represents the TEM image (a), EDX spectrum (b), and XRD pattern(c) obtained using the as-received γ-Fe₂O₃ (maghemite) magneticparticles.

FIG. 21 represents (a-c) the TEM images of nano-nano integratedHTN-γ-Fe₂O₃ magnetic nanocomposite particles, with the correspondingSAED patterns shown as the insets in (a,b), processed via anion-exchange mechanism operating under the dark-condition in an aqueoussolution. In (a) and (b,c), the amount of γ-Fe₂O₃ in the magneticnanocomposite is 50 wt. % and 10 wt. % respectively (balance being theweight of HTN).

FIG. 22 represents the EDX pattern of HTN-γ-Fe₂O₃ magnetic nanocompositeparticles processed via an ion-exchange mechanism operating under thedark-condition in an aqueous solution. The amount of γ-Fe₂O₃ in themagnetic nanocomposite is 10 wt. % (balance being the weight of HTN).

FIG. 23 represents the aqueous solution of MB dye (7.5 μM) (i) and theclear aqueous solution obtained after adsorbing the MB dye using theHTN-γ-Fe₂O₃ magnetic nanocomposite particles, which are settled at thebottom after the dye-adsorption process using a moderate externalmagnetic field (ii). The amount of γ-Fe₂O₃ in the magnetic nanocompositeis 10 wt. % (balance being the weight of HTN). M′ represents an externalmagnet and arrow indicates its position.

FIG. 24 represents the variation in the PL intensity associated with theformation of 2-hydroxyterepthalic acid as a function of UV-radiationexposure time as obtained for the magnetic γ-Fe₂O₃ nanoparticles (a),hydrothermally processed HTN (b), and commercially availablenanocrystalline anatase-TiO₂ (Central Drug House (CDH) (P) Ltd., NewDelhi, India) (c). The excitation wavelength is ˜315 nm.

FIG. 25 represents the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time as obtained forthe HTN-γ-Fe₂O₃ (5 wt. %) magnetic nano-composite particles under thedifferent test-conditions. The initial MB dye concentration varies as 30μM (i) and 100 μM (ii, iii). The graphs (ii) and (iii) are obtainedwithout and with the surface-cleaning treatment of HTN-γ-Fe₂O₃ (5 wt. %)magnetic nano-composite particles following the 1.2 mg·g⁻¹ of MB dyeadsorption on the surface as obtained in the graph (i).

FIG. 26 represents (a) typical TEM image of the Pt-depositednanocrystalline anatase-TiO₂ photocatalyst having the Pt/Ti mole-ratioof 5% and (b) the variation in the UV-visible absorption intensityobtained after the dye-adsorption experiments conducted using theindustry effluent samples containing the Red M5B reactive dye, asobserved for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nano-compositeparticles under the different test-conditions—(i) initial dye-solution,(ii) 1^(st) cycle of dye-adsorption, (iii) 2^(nd) cycle ofdye-adsorption, (iv-vi) 2^(nd) cycle of dye-adsorption following thesurface-cleaning treatment conducted using the Pt-depositednanocrystalline anatase-TiO₂ having the Pt/Ti mole-ratio of 1, 5, and10% respectively. In (a), the SAED pattern is shown as an inset.

FIG. 27 represents the TEM image (a) and EDX spectrum (b) obtained usingthe γ-Fe₂O₃-HTN-flyash magnetic nanocomposite. In (a), the inset showsthe corresponding SAED pattern. The HTN-γ-Fe₂O₃-flyash magneticcomposite particles contains ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balancebeing the weight of flyash particles).

FIG. 28 represents the digital photographs comparing the ability of avertically held magnet (M), producing a moderate magnetic field, to holdthe as-received flyash particles (a) and HTN-γ-Fe₂O₃-flyash magneticnanocomposite particles (b). The arrows at the left-hand bottom-cornerand right-hand upper-corner indicate the respective position ofas-received flyash particles (a) and HTN-γ-Fe₂O₃-flyash magneticnanocomposite particles (b). The HTN-γ-Fe₂O₃-flyash magnetic compositeparticles contains ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being theweight of flyash particles).

FIG. 29 represents (a) the variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time as obtained forthe as-received flyash particles (i) and HTN-γ-Fe₂O₃-flyash magneticcomposite particles (ii). The initial MB dye concentration and theinitial solution-pH are 7.5 μM and 7.5. (b) represents the aqueoussolution of MB dye (7.5 μM) (i) and the clear aqueous solution obtainedafter adsorbing the MB dye using the HTN-γ-Fe₂O₃-flyash magneticcomposite particles, which are settled at the bottom after thedye-adsorption process using a moderate external magnetic field (ii). M′represents an external magnet and arrow indicates its position. TheHTN-γ-Fe₂O₃-flyash magnetic composite particles contain ˜1 wt. % γ-Fe₂O₃and ˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyashparticles);

FIG. 30 represents the TEM image (a) and EDX spectrum (b) obtained usingthe (10 wt. %) HTN-SnO composite particles.

FIG. 31 represents schematically flyash particles beingsurface-sensitized by adsorbing metal-cations (Mn+) on their surfaces.

FIG. 32 represents schematically, the formation of composite particlesundergoing an ion-exchange mechanism, operating under the dark-conditionin an aqueous solution, with the metal-cations (M^(n+)) which arepre-adsorbed on the surface of flyash particles.

FIG. 33 represents schematically replacing of pure semiconductor-oxidesnanotubes with those having surface-deposited magnetic nanoparticles orattached to the magnetic nanoparticles (shown as an oval-shapedark-color particle).

FIG. 34 represents schematically attaching or anchoringsemiconductor-oxides nanotubes to the surface of magnetic γ-Fe₂O₃nanoparticles via an ion-exchange mechanism operating under thedark-condition in an aqueous solution.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method involving both, thesurface-sensitization of flyash particles with the metal-cations(M^(n+)) or protons and an ion-exchange mechanism operating under thedark-condition in an aqueous solution, to process an innovative productconsisting of the semiconductor-oxides nanotubes-flyash compositeparticles (magnetic or non-magnetic), and a method to recycle thesecomposite particles in the dye-removal application thereof. The presentinvention also provides a method, involving an ion-exchange mechanismoperating under the dark-condition in an aqueous solution, to process aproduct consisting of the semiconductor-oxides nanotubes-metal oxidenanocomposite particles (magnetic or non-magnetic), by eliminating thesurface-sensitization step, wherein the metal-oxide is essentially theoxide of metal-cation (M^(n+)) which can surface-sensitize the flyashparticles; and a method to recycle the magnetic/non-magneticcomposite/nanocomposite particles in the dye-removal applicationthereof. In the present invention, the as-received flyash particles arefirst surface-sensitized by adsorbing the metal-cations (M^(n+)) ontheir surfaces, selected from the group consisting of Sn²⁺/Sn⁴⁺,Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁴⁺, and others, by stirring theflyash particles in an acidic aqueous solution of metal-salt selectedfrom the group consisting of chloride, nitrate, and sulfate-salts ofmetal-cations, as shown in FIG. 31.

The semiconductor-oxides nanotubes (HTN or ATN), which are processedseparately via the conventional hydrothermal technique followed by thetypical washing-cycles, are added to an acidic aqueous suspension ofsurface-sensitized flyash particles under the continuous overheadstirring. The nanotubes get deposited on the surface-sensitized flyashparticles under the dark-condition in an aqueous solution, forming thenano-micro integrated semiconductor-oxides nanotubes-flyash compositeparticles (non-magnetic). The formation of composite particles has beenattributed to the ability of semiconductor-oxides nanotubes to undergoan ion-exchange mechanism, operating under the dark-condition in anaqueous solution, with the metal-cations (M^(n+)) which are pre-adsorbedon the surface of flyash particles, as shown in FIG. 32.

The metal-cations (M^(n+)) may be replaced with the protons (H⁺) toattach semiconductor-oxides nanotubes to the surface of sensitizedflyash particles an ion-exchange mechanism operating under thedark-condition in an aqueous solution. The nanotubes are, hence,attached or anchored to the surface-sensitized flyash particlestypically at the short-edges (tube-openings) due to higher energy of thenanotube-edge relative to that of nanotube-surface. It is obvious thatreplacing the pure semiconductor-oxides nanotubes with those having thesurface-deposited (or surface-decorated) magnetic nanoparticles (metalor metal-oxide) or attached to the magnetic nanoparticles (metal ormetal oxide, shown as an oval-shape dark-color particle), thesemiconductor-oxides nanotubes-magnetic nanoparticles-flyash compositehaving the magnetic property can be produced, as shown in FIG. 33.

The semiconductor-oxides nanotubes-flyash composite particles (magneticor non-magnetic) (and also the as-received flyash particles) aresuitable for the removal of an organic synthetic-dye from an aqueoussolution via the surface-adsorption process, involving the ion-exchangeand electrostatic-attraction mechanisms operating in the dark-condition,and can be separated from the treated aqueous solution via thecentrifuging, gravity-settling, or magnetic separation. It is obviousthat the dye-adsorption capacity of semiconductor-oxidesnanotubes-flyash composite particles can be increased by using thenanotubes which are doped with the noble-metal(s) or surface-depositedwith the noble-metal nanoparticles, selected from the group consistingof Au, Ag, Pt, Pd, and other noble-metals. The previously adsorbed-dyecan be decomposed on the surface of semiconductor-oxidesnanotubes-flyash composite particles (non-magnetic) (or the as-receivedflyash particles), via an innovative method, by mixing them with theconventional magnetic photocatalyst, such as the nanocrystallineanatase-TiO₂-coated SiO₂/γ-Fe₂O₃ in an aqueous solution and thensubjecting the resulting aqueous suspension to the UV or solar-radiationexposure under the continuous overhead stirring. The photocatalyticactivity of semiconductor-oxides nanotubes and as-received flyashparticles under the UV or solar-radiation is relatively low. However,relatively large concentration of free-OH. are generated by thenanocrystalline magnetic photocatalyst particles under the similartest-conditions, which attack and degrade an organic synthetic-dyepreviously adsorbed on the surface of semiconductor-oxidesnanotubes-flyash composite particles (non-magnetic) (or the as-receivedflyash particles). After the decomposition of previously adsorbed-dye onthe surface (that is, the surface-cleaning treatment), thesemiconductor-oxides nanotubes-flyash composite particles (non-magnetic)(or the as-received flyash particles) and the magnetic photocatalyst canbe separated from their mechanical mixture using a moderate externalmagnetic field. The surface-cleaned semiconductor-oxidesnanotubes-flyash composite particles (non-magnetic) (or the as-receivedflyash particles) can be recycled for the next-cycle of dye-adsorptionconducted in the dark-condition. It is obvious that for the recycling ofnon-magnetic semiconductor-oxides nanotubes-flyash composite particles(or the as-received flyash particles), in the dye-removal application,the nanocrystalline photocatalyst particles must be magnetic for theireffective magnetic separation after the dye-decomposition under anexposure to the UV or solar-radiation. It is also obvious that for therecycling of magnetic semiconductor-oxides nanotubes-flyash compositeparticles, the nanocrystalline photocatalyst particles must benon-magnetic such as the nanocrystalline anatase-TiO₂. Moreover, themagnetic photocatalyst, used for the recycling of semiconductor-oxidesnanotubes-flyash composite particles (or the as-received flyashparticles), may contain the magnetic component selected from the groupof γ-Fe₂O₃, CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Co, Fe, Ni, and other magneticmaterials, and the photocatalyst component selected from the group ofnanocrystalline semiconductor materials consisting of TiO₂, ZnO, CdS,ZnS, and others, in the undoped or doped form, without or with thesurface-modifications including the deposition of noble-metal or foreignmetal-oxide nanoparticles. It is also obvious that if the photocatalystcomponent of the magnetic photocatalyst is doped with the non-metalssuch as C, N, S, and others, an exposure to the fluorescent orvisible-radiation may also be used to generate the large concentrationof free-OH. for decomposing the previously adsorbed-dye on the surfaceof semiconductor-oxides nanotubes-flyash composite particles. To producethe semiconductor-oxides nanotubes-metal oxide nanocomposite particles(magnetic or non-magnetic), it is obvious that the flyash particles mustbe replaced with the metal-oxide nanoparticles. However, if themetal-oxide is the oxide of metal-cation (M^(n+)), which cansurface-sensitize the flyash particles, wherein the metal-oxide isselected from the group consisting of γ-Fe₂O₃, SnO/SnO₂, PbO, ZnO, CuO,MnO, TiO₂, and others, then the surface-sensitization is not anessential step and can be eliminated. Typically, to produce a magneticnanocomposite via an ion-exchange mechanism operating under thedark-condition in an aqueous solution, the as-received flyash particlesare replaced with the nanocrystalline magnetic metal-oxide particlessuch as γ-Fe₂O₃ (maghemite). Since γ-Fe₂O₃ contains Fe²⁺/Fe³⁺ cationswhich can surface-sensitize the as-received flyash particles, thesurface-sensitization step can be eliminated for attaching or anchoringthe semiconductor-oxides nanotubes to the surface of magnetic γ-Fe₂O₃nanoparticles via an ion-exchange mechanism operating under thedark-condition in an aqueous solution, as shown in FIG. 34.

The semiconductor-oxides nanotubes are attached or anchored to thesurface of metal-oxide, such as γ-Fe₂O₃, nanoparticles typically at theshort-edges (tube-openings) due to higher energy of the nanotube-edgerelative to that of nanotube-surface. The nano-nano integratedsemiconductor-oxide nanotubes-metal oxide nanocomposite particles, thusproduced, are also suitable for the removal of an organic synthetic-dyefrom an aqueous solution via the surface-adsorption process, involvingthe ion-exchange and electrostatic-attraction mechanisms, operating inthe dark-condition. Typically, the semiconductor-oxidesnanotubes-γ-Fe₂O₃ magnetic nanocomposite particles find application forthe removal of an organic synthetic-dye from an aqueous solution via thesurface-adsorption process, involving the ion-exchange andelectrostatic-attraction mechanisms operating in the dark-condition, andcan be separated from the treated aqueous solution using an externalmagnetic field. The previously adsorbed-dye can be decomposed on thesurface of semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocompositeparticles by mixing them with the non-magnetic nanocrystalline particlesof anatase-TiO₂ photocatalyst in an aqueous solution and then subjectingthe resulting suspension to the UV or solar-radiation under thecontinuous overhead stirring. It is to be noted that the purenanocrystalline anatase-TiO₂ used for the surface-cleaning treatment mayget attached to (or anchored to) the HTN-γ-Fe₂O₃ magnetic nanocompositeparticles via an ion-exchange mechanism operating the under thedark-condition in an aqueous solution. This makes the magneticseparation of photocatalyst particles from the magnetic nanocompositeparticles difficult after the surface-cleaning treatment. To avoid thissituation, the pure nanocrystalline anatase-TiO₂ is not utilized as aphotocatalyst but surface-modified to avoid the operation ofion-exchange mechanism while maintaining (or even increasing) the highconcentration of free-OH. produced under the UV or solar-radiationexposure. The noble-metal-deposited (selected from the group of Pt, Au,Pd, Ag, and others) nanocrystalline anatase-TiO₂ effectively serves boththe purposes. The photocatalytic activity of semiconductor-oxidesnanotubes and γ-Fe₂O₃ magnetic nanoparticles under the UV orsolar-radiation is relatively low. The photocatalytic activity ofsemiconductor-oxides nanotubes, under the UV or solar-radiation, isfurther decreased if the previously adsorbed-dye exists on theirsurfaces. However, relatively large concentration of free-OH. aregenerated by the non-magnetic noble-metal deposited nanocrystallineanatase-TiO₂ photocatalyst particles under the UV or solar-radiation,which attack and degrade the organic synthetic-dye previously adsorbedon the surface of semiconductor-oxides nanotubes-γ-Fe₂O₃ magneticnanocomposite particles. After the decomposition of previouslyadsorbed-dye on the surface (that is, the surface-cleaning treatment),the semiconductor-oxides nanotubes-γ-Fe₂O₃ magnetic nanocompositeparticles and the non-magnetic noble-metal deposited nanocrystallineanatase-TiO₂ photocatalyst particles can be separated from theirmechanical mixture using a moderate external magnetic field. Thesurface-cleaned semiconductor-oxides nanotubes-γ-Fe₂O₃ magneticnanocomposite particles can be recycled for the next-cycle ofdye-adsorption conducted in the dark-condition. It is obvious that forthe magnetic nanocomposite particles, the nanocrystalline photocatalystparticles must be non-magnetic, which may be selected from the groupconsisting of the nanocrystalline (˜3-30 nm) semiconductor materialssuch as TiO₂, ZnO, CdS, ZnS, and others. It is also obvious that for therecycling of semiconductor-oxides nanotubes-metal oxide nanocompositeparticles (non-magnetic), the nanocrystalline photocatalyst particlesmust be magnetic, that is magnetic photocatalyst, which may contain themagnetic component selected from the group of γ-Fe₂O₃, CoFe₂O₄, NiFe₂O₄,MnFe₂O₄, Co, Fe, Ni and other magnetic materials, and the photocatalystcomponent selected from the group of nanocrystalline (˜3-30 nm)semiconductor materials consisting of TiO₂, ZnO, CdS, ZnS, and others,in the undoped or doped form, without or with the surface-modificationsincluding the deposition of noble-metal or foreign metal-oxidenanoparticles. It is also obvious that if the photocatalyst particles(magnetic or non-magnetic) are doped with the non-metals such as C, N,S, and others, an exposure to the fluorescent or visible-radiation maybe used to generate the large concentration of free-OH. for decomposingthe previously adsorbed-dye on the surface of semiconductor-oxidesnanotubes-metal Oxide nanocomposite particles. It is obvious that thepreviously adsorbed-dye on the surface of semiconductor-oxidesnanotubes-flyash composite particles (magnetic or non-magnetic) and thesemiconductor-oxides nanotubes-metal oxide nanocomposite particles(magnetic or non-magnetic) can be decomposed via the Fenton-likereactions by stirring them in the H₂O₂ solution (3-100 wt. %) typicallyin the dark-condition. Under this situation, the use of both thephotocatalyst particles and an exposure to the UV, solar, fluorescent,or visible-radiation is not essential for the surface-cleaningtreatment; that is, the latter can be conducted in the dark-condition.It is also obvious that the different composite products of thisinvention can also be used to form a static-bed for the dye-removalapplication, wherein the static-bed is formed using a mechanical mixtureof non-magnetic flyash-based composite particles and the magneticphotocatalyst particles or using a mechanical mixture of magneticnanocomposite particles and the non-magnetic photocatalyst particles.After the dye-adsorption obtained through the passage of aqueousdye-solution thorough the bed-column, the static-bed having thesurface-adsorbed dye may be exposed to the UV, solar, fluorescent, orvisible-radiation while simultaneously passing an aqueous solutionthrough the bed-column to decompose the previously adsorbed-dye on thesurface. The previously adsorbed-dye on the surface of static-bed mayalso be decomposed, typically in the dark-condition, by passing the H₂O₂solution (3-100 wt. %) through the bed-column. If the static-bed isformed using only the flyash-based composite products with the absenceof photocatalyst particles, then the previously adsorbed-dye on thesurface of static-bed may be decomposed, typically in thedark-condition, by passing the H₂O₂ solution (3-100 wt. %) through thebed-column. The present invention provides an innovative method, whichinvolves the surface-sensitization step, for the processing of aninnovative product consisting of the nano-micro integratedsemiconductor-oxides nanotubes-flyash composite particles (magnetic ornon-magnetic), via an ion-exchange mechanism operating under thedark-condition in an aqueous solution, and the industrial dye-removalapplication involving their recycling via an innovative method thereof;50-250 ml acidic aqueous solution having the initial solution-pH in therange of ˜1-2, adjusted using the 1 M HCI solution, is first prepared;5-60 g·l⁻¹ of metal-salt is dissolved in the acidic aqueous solution,wherein the metal-salt is selected from the group consisting ofchloride, nitrate, and sulfate-salts of Sn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺,Zn²⁺, Cu²⁺, Mn²⁺, Ti⁺, and other metal-cations (in general, M^(n+)); thedissolution time of metal-salt in the acidic aqueous solution is variedin the range of 30 min-5 h under the continuous overhead stirring; 1-10g of as-received flyash particles having the spherical morphology anddiameter in the range of 0.5-100 μm are then suspended in the acidicaqueous solution of metal-salt; the resulting suspension is stirredcontinuously using an overhead stirrer for 30 min-5 h for adsorbing themetal-cations (Mn⁺) on the surface of as-received flyash particles(surface-sensitization); the semiconductor-oxides nanotubes such as HTNor ATN are processed separately via the conventional hydrothermal methodin combination with either conventional sol-gel or acetic acid (CH₃COOH)modified sol-gel method; 0.05-3 g of HTN or ATN are then added to theabove suspension of surface-sensitized flyash particles under thecontinuous overhead stirring; the suspension is then sonicated for 5-30min and then stirred continuously using an overhead stirrer for 30 min-5h for the adsorption of semiconductor-oxides nanotubes on thesurface-sensitized flyash particles via an ion-exchange mechanismoperating under the dark-condition in an aqueous solution; thenano-micro integrated semiconductor-oxides nanotubes-flyash compositeparticles (non-magnetic), thus formed, are separated using a centrifugeoperated at 2000-4000 rpm, washed multiple-times using 50-250 mldistilled-water till the solution-pH of filtrate remains unchanged orconstant, separated using a centrifuge operated at 2000-4000 rpm, andthen dried in an oven at 70-90° C. for 10-15 h. The various parameterssuch as the dissolution time and initial concentration of metal-salt,the contact time of aqueous solution of metal-salt with the as-receivedflyash particles and that of the semiconductor-oxides nanotubes with thesurface-sensitized flyash particles are optimized for the as-receivedflyash particles surface-sensitized with the Sn²⁺ cations. Theas-received flyash particles or the nano-micro integratedsemiconductor-oxides nanotubes-flyash composite particles (non-magnetic)are suitable for the removal of an organic synthetic-dye from an aqueoussolution via the surface-adsorption process involving the ion-exchangeand electrostatic-attraction mechanisms operating in the dark-condition.The dye-adsorption measurements are conducted in the dark-conditionusing the methylene blue (MB) as a model catalytic dye-agent. The effectof Ag-doping (Ag/Ti ratio is varied as 0, 1, and 5%) on thedye-adsorption capacity of pure-ATN and ATN-flyash composite particlesis shown within the initial MB dye concentration range of 5-100 μM atthe initial solution-pH within the range of ˜2.5-11. In order todecompose the previously adsorbed-dye on the surface of as-receivedflyash particles or the semiconductor-oxides nanotubes-flyash compositeparticles (non-magnetic) and to recycle them as catalyst for thenext-cycles of dye-adsorption conducted in the dark-condition, 0.1-5 gof as-received flyash particles or the semiconductor-oxidesnanotubes-flyash composite particles (non-magnetic) having thesurface-adsorbed MB dye (0.1-10 mg·g⁻¹) are suspended in 50-250 mlaqueous solution under the continuous overhead stirring; 0.1-5 g ofconventional magnetic photocatalyst (typically, the nanocrystalline(˜3-30 nm) anatase-TiO₂-coated SiO₂/γ-Fe₂O₃ magnetic particles(processed separately via the combination of modified-Stober andconventional sol-gel methods) are then suspended under the continuousoverhead stirring; the resulting suspension is sonicated for 5-30 minand then stirred continuously using an overhead stirrer under the UV orsolar-radiation exposure for 1-10 h; the mixture of as-received flyashparticles or the semiconductor-oxides nanotubes-flyash compositeparticles (non-magnetic) and the magnetic photocatalyst particles areseparated using a centrifuge operated at 2000-4000 rpm, washedmultiple-times using 50-250 ml distilled-water till the solution-pH ofthe filtrate remained unchanged or neutral; the non-magnetic as-receivedflyash particles or the semiconductor-oxides nanotubes-flyash compositeparticles and the magnetic photocatalyst particles are separated using amoderate external magnetic field and then dried in an oven at 70-90° C.for 10-15 h; the dried as-received flyash particles or thesemiconductor-oxides nanotubes-flyash composite particles (non-magnetic)are recycled for the next-cycles of, dye-adsorption conducted in thedark-condition; the nano-micro integrated HTN-flyash composite particlesare also processed by replacing the Sn²⁺ cations with the Fe³⁺ cationsas the surface-sensitizer and iron(III) nitrate (Fe(NO₃)₃.6H₂O) as ametal-salt instead of SnCl₂; the amount of MB dye-adsorbed by thenano-micro integrated HTN-flyash composite particles, processed usingthe Fe³⁺ cations as the surface-sensitizer, is compared with thatadsorbed using the as-received flyash particles at the initial MBconcentration within the range of 7.5-100 μM and the initial solution-pHof 2.5-11; the nano-micro integrated HTN-flyash composite particles arealso processed by replacing the Sn²⁺ cations with the H⁺ ions as thesurface-sensitizer (that is, without the addition of a metal-salt, andhence, eliminating the dissolution time); the amount of MB adsorbed bythe nano-micro integrated HTN-flyash composite particles, processedUsing the H⁺ ions as the surface-sensitizer, is compared with thatadsorbed by the as-received flyash particles at the initial MBconcentration within the range of 7.5-100 μM and the initial solution-pHof 2.5-11. The present invention also provides an innovative methodwithout involving the surface-sensitization step for the processing ofan innovative product, consisting of the nano-nano integratedsemiconductor-oxides nanotubes-metal oxide nanocomposite particles(magnetic or non-magnetic), via an ion-exchange mechanism operatingunder the dark-condition in an aqueous solution, and the industrialdye-removal application involving their recycling via an innovativemethod thereof; the metal-oxide nanoparticles are selected from thegroup of metal-oxides, such as γ-Fe₂O₃ (magnetic), SnO/SnO₂, PbO, ZnO,CuO, MnO, TiO₂, and others, wherein the metal-oxide is essentially theoxide of metal-cation (M^(n+)) which can surface-sensitize the surfaceof flyash particles; the semiconductor-oxides nanotubes-γ-Fe₂O₃ magneticnanocomposite particles are processed via an ion-exchange mechanismoperating under the dark-condition in an aqueous solution without theinvolvement of surface-sensitization step since Fe²⁺/Fe³⁺ ions withinthe γ-Fe₂O₃ magnetic nanoparticles can surface-sensitize the as-receivedflyash particles; 50-250 ml aqueous suspension is first prepared bysuspending 0.1-0.9 g of γ-Fe₂O₃ magnetic nanoparticles are dispersed inan aqueous solution having the neutral solution-pH (˜6.5-7.5) under thecontinuous overhead stirring; 0.1-0.9 g of HTN is added to thissuspension under the continuous overhead stirring; the resultingsuspension containing total 1 g of solid particles added is sonicatedfor 5-30 min and then stirred in the dark-condition for 1-10 h using anoverhead stirrer; the HTN-γ-Fe₂O₃ magnetic nanocomposite particles, thusformed via an ion-exchange mechanism operating under the dark-conditionin an aqueous solution, are separated using the moderate externalmagnetic field and washed multiple-times using the distilled-water tillthe solution-pH of filtrate remains unchanged or neutral; the magneticnanocomposite particles are separated again using the moderate externalmagnetic field and dried in an oven at 70-90° C. for 10-15 h.

0.1-5.0 g of semiconductor oxide nanotubes-γ-Fe₂O₃ magneticnanocomposite particles, containing 5-90 wt. % of HTN, are then used forthe removal of MB dye (7.5-250 μM) from an aqueous solution (50-250 ml)via the surface-adsorption process, involving the ion-exchange andelectrostatic-attraction mechanisms operating in the dark-condition.

To decompose the previously adsorbed-dye on the surface and to recyclethem for the next-cycles of dye-adsorption conducted in thedark-condition, 0.1-5.0 g of magnetic HTN-γ-Fe₂O₃ nanocompositeparticles, containing 1-5 mg·g⁻¹ of MB dye adsorbed on the surface, aresuspended in 50-250 ml aqueous solution under the continuous overheadstirring; the non-magnetic Pt-deposited (Pt/Ti mole ratio within therange of 1-15%) nanocrystalline (˜10-30 nm) anatase-TiO₂ photocatalystparticles are then added to the above suspension under the continuousoverhead stirring; the resulting suspension is sonicated for 5-30 min,then exposed to the UV or solar-radiation for 30 min-5 h under thecontinuous overhead stirring; the mixture of surface-cleaned magneticnanocomposite particles and the non-magnetic Pt-depositednanocrystalline anatase-TiO₂ photocatalyst particles are separated usinga centrifuge operated at 2000-4000 rpm and washed multiple-times using50-250 ml distilled-water till the solution-pH of filtrate remainsunchanged; the surface-cleaned magnetic nanocomposite particles areseparated from the non-magnetic Pt-deposited nanocrystallineanatase-TiO₂ photocatalyst using an external magnetic field; theseparated powders are dried in an oven at 70-90° C. for 10-15 h; thesurface-cleaned magnetic nanocomposite particles are recycled for thenext-cycle of dye-adsorption conducted in the dark-condition and itsdye-adsorption behavior is compared with that of magnetic nanocompositeparticles having the surface-adsorbed MB dye which is not subjected tothe surface-cleaning treatment.

The present invention also provides an innovative method for theprocessing of an innovative product consisting of flyash-based magneticcomposite particles; the HTN-γ-Fe₂O₃ magnetic nanocomposite (5-90% HTN)is first processed, without involving the surface-sensitization step,via the ion-exchange mechanism operating under the dark-condition in anaqueous solution; the HTN-γ-Fe₂O₃ magnetic nanocomposite is thenattached or anchored to the flyash particles, surface-sensitized withSn²⁺ cations, via the ion-exchange mechanism operating under thedark-condition in an aqueous solution.

The non-obvious inventive step(s) of the present invention with respectto the prior art are as follows.

-   (1) The addition of hydrothermally processed semiconductor-oxide    nanotubes, such as the HTN or ATN, in the aqueous suspension of    flyash particles having the pre-adsorbed surface metal-cations.-   (2) The formation of micro-nano integrated semiconductor-oxide    nanotubes-flyash composite particles via an ion-exchange mechanism    operating under the dark-condition in an aqueous solution.-   (3) The use of micro-nano integrated semiconductor-oxide    nanotubes-flyash composite particles (or the as-received flyash    particles) in the dye-removal application via the surface-adsorption    process involving the ion-exchange and electrostatic-attraction    mechanisms operating in the dark-condition, wherein the composite    particles (or the as-received flyash particles) are recycled by    decomposing the previously adsorbed-dye on the surface via an    innovative process involving the use of combination of a magnetic    photocatalyst and an exposure to the UV or solar-radiation.-   (4) The addition of hydrothermally processed semiconductor-oxide    nanotubes, such as the HTN or ATN, in an aqueous suspension of    metal-oxide particles at the neutral solution-pH, wherein the    metal-oxide nanoparticles contain the metal-cations (M^(n+)) which    can surface-sensitize the flyash particles. This results in the    formation of semiconductor-oxide nanotubes-metal oxide    nano-composite (magnetic or non-magnetic) particles without    involving the surface-sensitization step.-   (5) The formation of nano-nano integrated semiconductor-oxide    nanotubes-metal oxide nanocomposite particles (magnetic or    non-magnetic) via an ion-exchange mechanism operating under the    dark-condition in an aqueous solution without involving the    surface-sensitization step.-   (6) The use of nano-nano integrated semiconductor-oxide    nanotubes-metal oxide nanocomposite particles (magnetic) in the    dye-removal application via the surface-adsorption process involving    the ion-exchange and electrostatic attraction mechanisms operating    in the dark-condition, wherein the magnetic nanocomposite particles    are recycled by decomposing the previously adsorbed-dye on the    surface via an innovative process involving the use of combination    of non-magnetic noble-metal-deposited (including Pt, Au, Pd, Ag, and    others) photocatalyst, such as nanocrystalline anatase-TiO₂, and    exposure to the UV or solar-radiation.-   (7) The adsorption of semiconductor-oxide nanotubes, which are    attached to (or anchored to) the magnetic metal-oxide nanoparticles,    on the surface-sensitized flyash particles via an ion-exchange    mechanism operating under the dark-condition in an aqueous solution    resulting in the formation of flyash-based magnetic composite    particles.

The novelty of the present invention with respect to the prior art is asfollows.

-   (1) The formation of micro-nano integrated semiconductor-oxide    nanotubes-flyash composite particles via an ion-exchange mechanism    operating under the dark-condition in an aqueous solution.-   (2) The formation of nano-nano integrated semiconductor-oxide    nanotubes-metal oxide nanocomposite particles via an ion-exchange    mechanism operating under the dark-condition in an aqueous solution    by eliminating the surface-sensitization step.-   (3) The recycling of semiconductor-oxide nanotubes-flyash composite    particles (non-magnetic) in the dye-removal application involving    the use of a magnetically separable and photocatalytically active    magnetic photocatalyst.-   (4) The recycling of semiconductor-oxide nanotubes-metal oxide    nanocomposite particles (magnetic) in the dye-removal application    involving the use of non-magnetic and photocatalytically active    noble-metal-deposited nanocrystalline anatase-TiO₂ particles.-   (5) The formation of micro-nano integrated semiconductor-oxide    nanotubes-metal oxide-flyash composite particles (magnetic) via an    ion-exchange mechanism operating under the dark-condition in an    aqueous solution.-   (6) The use of micro-nano integrated semiconductor-oxide    nanotubes-metal oxide-flyash composite particles (magnetic) in the    application related to the dye-removal via the surface-adsorption    process involving the ion-exchange and electrostatic attraction    mechanisms; and their recycling involving the use of non-magnetic    and photocatalytically active noble-metal-deposited nanocrystalline    anatase-TiO₂ particles.

EXAMPLES

The following examples are given by way of illustration therefore shouldnot be construed to limit the scope of the invention.

Example-1

In this example, HTN are processed via the conventional hydrothermalmethod. 3 g of as-received nanocrystalline anatase-TiO₂ (Central DrugHouse (CDH) (P) Ltd., New Delhi, India) is suspended in a highlyalkaline aqueous solution, containing 10 M NaOH (Assay 97%, S.D.Fine-Chem Ltd., Mumbai, India), filled up to 84 vol. % of aTeflon-beaker placed in a stainless-steel (SS 316) vessel of 200 mlcapacity. The process is carried out with continuous stirring in anautoclave (Amar Equipment Pvt. Ltd., Mumbai, India) at 120° C. for 30 hunder an autogenous pressure. The autoclave is allowed to cool naturallyto room temperature and the hydrothermal product is separated bydecanting the top solution. The initial product is subjected to typicalwashing-cycles with the first-cycle of washing conducted using 100 ml of1 M HCI solution (35 wt. %, Qualigens Fine Chemicals, India) at 25° C.for 1 h followed by that using 100 ml of distilled-water for 1 h. Theproduct obtained is then subjected to the second washing-cycleconsisting of washing using 100 ml of 1 M HCI at 25° C. for 1 h and thenmultiple times (#8) using 100 ml of distilled-water at 25° C. for 1 htill the pH (Hanna HI 2210 Bench Top, Sigma-Aldrich, India) of thefiltrate became almost constant or neutral. The washed-product is thenseparated from the solution using a centrifuge (Hettich EBA 20,Sigma-Aldrich, India) and dried in an oven at 80° C. for 12 h to obtainthe hydrothermally processed HTN.

The TEM images of as-received flyash particles (National Thermal PowerCorporation (NTPC), Ramagundam, India) and the hydrothermally processedHTN are presented in FIGS. 1(a) and 1(b). The as-received flyashparticles are spherical with the size in the range of 0.5-2 μM. Theaverage length, internal and outer diameters, and wall-thickness of HTNare 75, 2.3, 4.2, and 0.96 nm respectively. The powder XRD patterns ofas-received flyash and HTN are presented in FIGS. 2(a) and 2(b). In FIG.2(a), the major-peaks of SiO₂, Al₂O₃, and Fe₂O₃ have been identifiedafter comparison with the JCPDS card nos. 83-2465, 78-2426, and 84-0311respectively. In FIG. 2(b), the XRD pattern has been identified to besimilar to that of H₂Ti₃O₇ or the lepidocrocite-type hydrogen titanate(H_(x)Ti_(2-x/4γx/4)O₄.H₂O, where x≈0.7, γ: vacancy). The EDX analysesof as-received flyash and HTN are presented in FIGS. 3(a) and 3(b). InFIG. 3(a), Si, Al, Ca, Fe are observed with the trace amount of Ti (Cuis originating from the Cu-grid used for the TEM analysis). Thissuggests that the as-received flyash particles are composed of mixtureof SiO₂, Al₂O₃, Fe₂O₃, CaO, with the trace amount of TiO₂. On the otherhand, in FIG. 3(b), only Ti and O are seen as the major elements withinthe HTN. By merely stirring the aqueous suspension of mechanically mixedhydrothermally processed HTN and the as-received flyash particles, theformer could not be adsorbed on (or attached to or anchored to) thesurface of latter. In order to deposit HTN on the surface of as-receivedflyash particles, the following procedure has been adopted. 40 g·l⁻¹ oftin(II) chloride (SnCl₂, 97%, S.D. Fine-Chem, Mumbai, India) isdissolved in 100 ml of distilled-water having the initial solution-pH˜1.5 which is adjusted using the 1 M HCI solution under the continuousoverhead stirring (Eurostar Digital, IKA, Germany) at 25° C. Thedissolution time of SnCl₂ is 2 h. 1.0 g of as-received flyash particlesis added to this clear-solution and the resulting suspension is stirredcontinuously at 25° C. for 4 h using an overhead stirrer to obtain theSn²⁺-sensitized flyash particles. 0.1 g of hydrothermally processed HTNare then added and the resulting suspension is sonicated (BandelinUltrasonic Bath, Aldrich-Labware, Bangalore, India) for 10 min followedby continuous stirring using an overhead stirrer at 25° C. for 4 h. TheHTN are adsorbed on the surface of Sn²⁺-sensitized flyash particles viaan ion-exchange mechanism operating under the dark-condition in anaqueous solution. The resulting micro-nano integrated HTN-flyashcomposite particles (non-magnetic) are separated from the aqueoussolution using a centrifuge (Hettich EBA 20, Sigma-Aldrich, India)operated at 3000 rpm. The separated HTN-flyash composite particles arethen washed using 100 ml of distilled-water for 1 h multiple-times tillthe pH of filtrate remains unchanged or neutral. The washed compositeparticles are then dried in an oven at 80° C. for 12 h to obtain theHTN-flyash composite particles (non-magnetic) containing 69 wt. % Sn and3 wt. % HTN (balance being the weight of flyash particles). The highmagnification TEM images of the surface of as-received flyash particleand the interphase boundary within the micro-nano integrated HTN-flyashcomposite particle (non-magnetic) are shown in FIGS. 4(a) and 4(b). Thesurface of as-received flyash particle at very high magnificationappears to be plain and featureless, FIG. 4(a). In FIG. 4(b), theinterface boundary of HTN-flyash composite particle (non-magnetic) isclearly visible. The HTN are seen to be attached to or anchored to thesurface of Sn²⁺-sensitized flyash particles typically at the short-edges(or tube-openings) resulting in the formation of micro-nano integratedHTN-flyash composite particles (non-magnetic). The mechanism of HTNadsorption on the surface of Sn²⁺-sensitized flyash particles is anion-exchange process operating under the dark-condition in an aqueoussolution. Since the as-received flyash particles are stirred in anacidic SnCl₂ solution, large amount of Sn²⁺ ions are first adsorbed onthe surface of as-received flyash particles. When the hydrothermallyprocessed HTN are added to an aqueous suspension containing theSn²⁺-sensitized flyash particles, the former tend to pick-up the Sn²⁺ions, which are pre-adsorbed on the surface of flyash particles, for anexchange with the H⁺ ions in their structure. However, since the Sn²⁺ions are already chemisorbed on the surface of as-received flyashparticles and cannot leave the surface, the HTN get adsorbed on thesurface of Sn²⁺-sensitized flyash particles. Since the nanotubes-edgehas higher energy than that of nanotubes-surface, the anchoring of HTNon the surface of Sn²⁺-sensitized flyash particles primarily occurs atthe shorter-edges (or the nanotubes-openings), FIG. 4(b), instead ofalong the length of nanotubes. The XRD pattern, FIG. 5(a), and the EDXspectrum, FIG. 5(b), further confirm the formation of HTN-flyashcomposite particles via an ion-exchange mechanism operating under thedark-condition in an aqueous solution.

Example-2

In this example, the values of different parameters used for theprocessing of micro-nano integrated HTN-flyash composite particles areidentical with those already described in the Example 1 except for thefollowing changes. The initial concentration of SnCl₂ and itsdissolution time are varied as 5, 20, 40, and 60 g·l⁻¹ and 30 min, 2 and4 h respectively. With these processing parameters, the HTN-flyashcomposite particles (non-magnetic) containing 44, 53, 69, and 77 wt. %Sn and 2, 3, 4, and 7 wt % HTN respectively (balance being the weight offlyash particles) are obtained for the complete dissolution of SnCl₂(dissolution time of 2 and 4 h).

The HTN-flyash composite particles processed under these conditions areutilized in the dye-adsorption experiments which are conducted at theneutral initial solution-pH of ˜7.5 and in the dark-condition using theMB (methylene blue) as a model catalytic dye-agent. 125 ml aqueoussolution is prepared by dissolving 15 μM of MB dye. 4.0 g·l⁻¹ ofHTN-flyash composite particles are then dispersed in the MB dye solutionand the resulting suspension is stirred continuously in thedark-condition for 180 min using an overhead stirrer. 8 ml aliquot isseparated after each 10 or 30 min time interval for obtaining anabsorption spectrum, using the UV-visible absorption spectrophotometer(UV-2401 PC, Shimadzu, Japan), of the filtrate obtained after separatingthe HTN-flyash composite particles using a centrifuge. The normalizedconcentration of surface-adsorbed MB dye is calculated using theequation of form,

$\begin{matrix}{{{\%\mspace{14mu}{MB}_{adsorbed}} = {( \frac{C_{0} - C_{t}}{C_{0}} )_{MB} \times 100}},} & (1)\end{matrix}$which is equivalent of the form,

$\begin{matrix}{{\%\mspace{14mu}{MB}_{adsorbed}} = {( \frac{A_{0} - A_{t}}{A_{0}} )_{MB} \times 100}} & (2)\end{matrix}$where, C₀ (mg·l⁻¹) and C_(t) (mg·l⁻¹) correspond to the initial MB dyeconcentration at the start and after the contact time t with thecorresponding absorbance of A₀ and A_(t).

The variation in the normalized concentration of surface-adsorbed MB dyeas a function of contact time, as obtained for the micro-nano integratedHTN-flyash composite particles, is presented in FIG. 6(a)-6(c) for thedifferent values of initial SnCl₂ concentration and its dissolutiontime. It is noted that, in general, the micro-nano integrated HTN-flyashcomposite particles show higher MB dye-adsorption on the surfacerelative to that of as-received flyash particles. The Sn²⁺-cationsremaining in the aqueous solution, typically at lower dissolution timeof SnCl₂ and its higher concentrations, are seen to affect theadsorption of HTN on the surface of Sn²⁺-sensitized flyash particles.The amount of MB dye adsorbed on the surface of micro-nano integratedHTN-flyash composite particles per unit mass of adsorbent (q_(e)) iscalculated using the relationship of form,

$\begin{matrix}{q_{e} = \frac{( {C_{0} - C_{e}} ) \times V}{m_{FA}}} & (3)\end{matrix}$where, C_(e) (mg·l⁻¹) is the MB dye concentration within the solution atthe equilibrium (that is, after the contact time of 180 min), V (I) theinitial volume of MB dye solution, and m_(HTN-FA) (g) the mass ofmicro-nano integrated HTN-FA composite particles used as dye-adsorbent.The obtained variation in q_(e) as a function of initial SnCl₂concentration, obtained for the different dissolution time of SnCl₂, ispresented in FIG. 7. The as-received flyash particles exhibit the q_(e)of 0.3 mg·g⁻¹. On the other hand, the maximum q_(e) (q_(m)) of 1.2mg·g⁻¹ is observed for the initial SnCl₂ concentration of 20 g·l⁻¹ andthe dissolution time of 2 h, which suggests 4 times increase in thedye-adsorption capacity, at the solution-pH of ˜7.5, due to thedeposition of hydrothermally processed HTN on the surface of as-receivedflyash particles via an ion-exchange mechanism operating in thedark-condition.

Thus, under the given test-conditions, the HTN-flyash compositeparticles exhibit higher MB dye adsorption capacity than that ofas-received flyash particles. The initial SnCl₂ concentration of 20g·l⁻¹ and the dissolution time of 2 h are determined to be the mostoptimum conditions leading to the maximum MB dye-adsorption on thesurface of HTN-flyash composite particles in the dark-condition. Sincein the literature, the capacity of as-received flyash particles for theadsorption of cationic MB dye is shown to be insensitive to the initialsolution-pH; while, that of hydrothermally processed HTN is shown to bedrastically enhanced and reached its maximum value at the initialsolution-pH of ˜10 (within the range of ˜2.5-11), it is obvious that thedifference in the dye-adsorption capacity of HTN-flyash compositeparticles (non-magnetic) and that of as-received flyash particles, atthe initial solution-pH of ˜10, would be the highest and more than thatobserved at the initial solution-pH of ˜7.5.

Example-3

In this example, the values of different parameters used for theprocessing of micro-nano integrated HTN-flyash composite particles areidentical with those already described in the Example 1 except for thefollowing changes. The initial SnCl₂ concentration is changed to 20g·l⁻¹. Both the contact time of SnCl₂ solution with the suspendedas-received flyash particles and that of the hydrothermally processedHTN with the Sn²⁺-sensitized flyash particles are varied in the range of1-4 h at the interval of 1 h. The HTN-flyash composite particlesprocessed under these conditions contain 53 wt. % Sn and 4 wt. % HTN(balance being the weight of flyash particles) and are utilized in theMB dye-adsorption experiments, conducted in the dark-condition, asdescribed above in the Example-2.

The obtained variation in the normalized concentration ofsurface-adsorbed MB dye as a function of contact time, as obtained forthe micro-nano integrated HTN-flyash composite particles, for thedifferent contact time of SnCl₂ solution with the as-received flyashparticles, FIG. 8(a), and that of the HTN with the Sn²⁺-sensitizedflyash particles, FIG. 8(b). It is noted that, for both the contact timewithin the range of 1-4 h, the MB dye adsorption remains almost 100%.Hence, within the investigated range of parameters, the contact time ofSnCl₂ solution with the as-received flyash particles and that of the HTNwith the Sn²⁺-sensitized flyash particles do not have any significanteffect on the amount of MB dye adsorbed on the surface of HTN-flyashcomposite particles.

Thus, considering that higher amount of HTN (greater than ˜10%) could beadsorbed on the surface of Sn²⁺-sensitized flyash particles, the optimumcontact time of SnCl₂ solution with the as-received flyash particles andthat of HTN with the Sn²⁺-sensitized flyash particles are determined tobe 4 h.

Example-4

In this example, the values of different parameters used for theprocessing of micro-nano integrated HTN-flyash composite particles areidentical with those already described in the Example 1 except that theinitial concentration of SnCl₂ is 20 g·l⁻¹. The HTN-flyash compositeparticles processed under these conditions contain 53 wt. % Sn and 4 wt.% HTN (balance being the weight of flyash particles) are utilized in theMB dye-adsorption experiments, conducted in the dark-condition, asdescribed earlier in the Example-2 except that the initial MB dyeconcentration is varied in the range of 15-90 μM.

The variation in the normalized concentration of surface-adsorbed MB dyeas a function of contact time, as obtained for the micro-nano integratedHTN-flyash composite particles, for the different initial MB dyeconcentration, is presented in FIG. 9(a). The amount of surface-adsorbedMB dye, at equilibrium, is seen to decrease with increasing initial MBdye concentration. The corresponding variation in q_(e) as a function ofinitial MB dye concentration is presented in FIG. 9(b). The maximumvalue of q_(e)(q_(m)), under these test-conditions, is noted be 4mg·g⁻¹, which is 13 times higher than that (0.3 mg·g⁻¹) of as-receivedflyash particles as determined under the similar test-conditions.

Thus, under the neutral initial solution-pH of ˜7.5, the dye-adsorptioncapacity of HTN-flyash composite particles is higher than that ofas-received flyash particles. It is obvious that by changing the initialsolution-pH within the range of 7.5-11, the dye-adsorption capacity ofHTN-flyash composite would be further enhanced (typically at the initialsolution-pH of ˜10) relative to that of as-received flyash.

Example-5

In this example, Ag-doped ATN with varying Ag/Ti mole-ratio (0, 1, and5%) are synthesized via the conventional hydrothermal method as alreadydescribed in the Example 1 except that the as-received anatase-TiO₂precursor is replaced with the Ag-doped anatase-TiO₂ processed withvarying Ag/Ti mole-ratio (0, 1, and 5%). The latter is processed via theconventional sol-gel method modified using the acetic acid (CH₃COOH) asa catalyst. The molar-ratio of CH₃COOH catalyst to Ti(OC₃H₅)₄ precursor,involved in the modified sol-gel process, is 10. The molar-ratio ofwater to alkoxide-precursor (R-value) is 90. The CH₃COOH-catalyst isessential in the modified sol-gel process to dissolve Ag into the TiO₂lattice without causing the anatase-to-rutile phase transformationduring the calcination treatment which is conducted at highertemperature at 600° C. for 2 h. In contrast to the formation of HTN asdescribed in the Example 1, the nanotubes formed in this example areobserved to be ATN, FIG. 10. The pure and Ag-doped ATN processed underthese conditions are then utilized in the MB dye-adsorption experiments,conducted in the dark-condition, as described earlier in the Example-2except that the initial MB dye concentration is varied in between7.5-250 μM.

The obtained variation in q_(e) as a function of initial MB dyeconcentration, as obtained using the pure and Ag-doped ATN with varyingAg/Ti mole-ratio (0, 1, and 5%), is presented in FIG. 11(a). The maximumvalue of q_(e) (q_(m)) is noted be 38 mg·g⁻¹ as observed for theAg-doped ATN having 1% Ag/Ti mole-ratio. Hence, it is obvious thatAg-doped ATN-flyash composite particles would exhibit higher MBdye-adsorption capacity than that of pure ATN-flyash compositeparticles, which is demonstrated in FIG. 11(b). Hence, the doping ofnoble-metal increases the dye-adsorption capacity of ATN (or HTN) andalso that of semiconductor-oxide nanotubes-flyash composite particles.

It is obvious that doping ATN or HTN with other noble-metals such as Au,Pt, and Pd as well as non-noble metals such as Gd, Zn, Mn, Cu, andothers, would also increase the dye-adsorption capacity of ATN (orHTN)-flyash composite particles. It is also obvious that the othertechniques including the surface-deposition of noble-metal catalystnanoparticles, such as Au, Pt, Pd, Ag, and others, on the surface of ATN(or HTN)-flyash composite particles would increase their dye-adsorptioncapacity.

Example-6

In this example, the anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magneticnanocomposite particles are first prepared via the modified-Stober andthe conventional sol-gel processes. To 2 g suspension of nanocrystallinemagnetic γ-Fe₂O₃ particles dispersed in 250 ml of ethanol, 14.5 ml oftetraethylorthosilicate (TEOS, 98%, Sigma-Aldrich, India) is slowlyadded and stirred for 1 h using an overhead stirrer. This is followed bythe drop-wise addition of mixture of 2.3 ml of NH₄OH (25 wt. %,Qualigens Fine Chemicals, India) and 63.4 ml of distilled-water and thesuspension is stirred for 12 h. The resulting product is collected viamagnetic separation (magnetic separator, Sigma-Aldrich Labware,Bangalore, India), washed first with 100 ml of ethanol and four timeswith distilled-water followed by drying in an oven at 80° C. for 12 h.By this process, the SiO₂/γ-Fe₂O₃ magnetic nanocomposite particlescontaining 66 wt. % SiO₂ and 34 wt. % γ-Fe₂O₃ are obtained.

In order to deposit the nanocrystalline anatase-TiO₂, 2 g ofSiO₂/γ-Fe₂O₃ magnetic nanoparticles are suspended in a solution of 0.5 gof titanium(IV) iso-propoxide (Ti(OC₃H₇)₄, 97%, Sigma-Aldrich,Bangalore, India) dissolved in 125 ml 2-propanol. To this suspension, asolution consisting 0.15 ml of distilled water (R=5, defined as theratio of molar concentration (final) of water to that ofalkoxide-precursor) dissolved in 125 ml of 2-propanol, was added dropwise. The resulting suspension is stirred for 12 h and the magneticallyseparated powder is then washed with 100 ml of 2-propanol and then driedin an oven at 80° C. overnight. The sol-gel process is repeated fortotal 5 cycles to control the thickness of amorphous-TiO₂ coating whichis then converted to anatase-TiO₂ via the calcination treatment (heatingrate=3° C.·min⁻¹) of the dried-powder conducted at 600° C. for 2 h. Theanatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic nanocomposite, thusobtained, contains 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and 49 wt. % SiO₂and is referred here as a “magnetic photocatalyst”. The correspondingSEM image, EDX spectrum, and XRD pattern of magnetic photocatalyst arepresented in FIG. 12(a)-12(c) respectively. Being magnetic in nature,the anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃ magnetic photocatalystcan be separated from an aqueous solution using an external magneticfield as demonstrated in FIG. 13. (Note: γ-Fe₂O₃ is a ferrimagneticmaterial with the saturation magnetization of 74 emu·g⁻¹).

The free-OH. trapping experiments are performed, using the terepthalicacid (TA, 98%, Sigma-Aldrich Chemicals, Bangalore, India), which areproduced under the continuous solar-radiation exposure of two separateaqueous suspensions containing the suspended particles of as-receivedflyash particles or the HTN-flyash composite particles (non-magnetic)and the magnetic photocatalyst. 500 μM of TA and 2000 μM of NaOH (assay97%, S.D. Fine-Chem, Mumbai, India) are first dissolved in 125 ml ofaqueous solution. Either 3.2 g·l⁻¹ of magnetic photocatalyst or 24 g·l⁻¹of as-received flyash particles (or HTN-flyash composite particles) aresuspended in this solution. The resulting suspension is stirredcontinuously using an overhead stirrer under the solar-radiationexposure for 5 h. The free-OH. produced by the as-received flyashparticles (or the HTN-flyash composite particles) and the magneticphotocatalyst particles, under the solar-radiation exposure, are trappedby the TA resulting in the formation of 2-hydroxyterephthalic acid. Thealiquots are collected at the 1 h time interval and the solid-particlesare separated using either a centrifuge or a magnetic separator. Thefiltrate is analyzed to obtain the photoluminescence (PL) spectra of2-hydroxyterephthalic acid which exhibits a characteristic PL peaklocated at ˜625 nm, the intensity of which is recorded as a function ofUV-radiation exposure time using the spectrofluorometer (Cary Eclipse,Varian, The Netherlands) at an excitation wavelength of ˜315 nm. Theintensity of PL peak is regarded as the measure of amount of free-OH.produced by the catalyst-particles at a given time under thesolar-radiation exposure.

The variation in the PL intensity of 2-hydroxyterepthalic acid as afunction of solar-radiation exposure Time, as observed for theas-received flyash particles (or HTN-flyash composite particles) and themagnetic photocatalyst, is presented in FIGS. 14(a) and 14(b). It isobserved that, for both the samples, the concentration of free-OH.produced increases with increasing solar-radiation exposure time.However, the comparison shows that the as-received flyash particles (orHTN-flyash composite particle) do not generate appreciable amount offree-OH. under the solar-radiation exposure. On the other hand, largeamount of free-OH. are produced by the magnetic photocatalyst. Thissuggests that the magnetic photocatalyst should possess betterphotocatalytic activity under the solar-radiation exposure than that ofthe as-received flyash particles (or HTN-flyash composite particles).Since the latter has relatively higher dye-adsorption capacity under thedark-condition, it appears that the previously adsorbed-dye can beeasily decomposed on the surface on as-received flyash particles (orHTN-flyash composite particles) by the attack of free-OH. produced bythe magnetic photocatalyst provided both the materials are mechanicallymixed and suspended in the same aqueous dye-solution which is exposed tothe solar-radiation. It further appears that from the mechanical mixtureof as-received flyash particles (or HTN-flyash composite particles) andthe magnetic photocatalyst, both the powders can be separated from theirmechanical mixture using an external magnetic field. This allows therecycling of as-received flyash particles (or HTN-flyash compositeparticles) for the next-cycle of dye-adsorption conducted in thedark-condition using the magnetic photocatalyst for theirsurface-cleaning treatment.

In order to demonstrate this, the as-received flyash particles are usedfor the multiple MB dye-adsorption cycles (#4) conducted in thedark-condition. All experimental parameters used for thesedye-adsorption measurements are identical to those described earlier theExample 2 except for the following changes. The initial concentration ofMB dye and that of adsorbent are 7.5 μM and 24 g·l⁻¹. In order to removethe previously adsorbed MB dye from the surface, 3 g of as-receivedflyash particles with the surface-adsorbed MB dye (0.24 mg·g⁻¹), asobtained after the cycle-4 of dye-adsorption as mentioned above, isfirst suspended in 125 ml of distilled-water under continuous overheadstirring. Then, 1 g of anatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃magnetic photocatalyst (containing 26 wt. % TiO₂, 25 wt. % γ-Fe₂O₃, and49 wt. % SiO₂) is added to the above suspension under the continuousoverhead stirring. The resulting suspension is stirred continuouslyunder the solar-radiation exposure for 6 h, which results in thedecomposition of MB dye adsorbed on the surface of as-received flyashparticles due to the large concentration of free-OH. radicals producedby the magnetic photocatalyst under these test-conditions. This is thenfollowed by the separation of surface-cleaned non-magnetic flyashparticles mixed with the magnetic photocatalyst particles using acentrifuge operated at 3000 rpm and the washing of mixed solid particlesusing 100 ml of distilled-water for 1 h multiple-times till the pH offiltrate remains unchanged or neutral. The magnetic and non-magneticcomponents are then separated using an external magnetic field and driedin an oven at 80° C. for 12 h. The surface-cleaned flyash particles,hence, could be recycled for the next-cycle of dye-adsorption conductedin the dark-condition as demonstrated in FIG. 15. The q_(e) is noteddecrease with increasing number of dye-adsorption cycles conducted inthe dark-condition. The surface-cleaning treatment, as described above,is applied after the dye-adsorption cycle-4, to decompose the previouslyadsorbed-dye from the surface, which results in a drastic increase inthe dye-adsorption capacity of as-received flyash particles which iscomparable with that observed in the cycle-1.

Thus, the recycling of as-received flyash particles is successfullyachieved here by decomposing the previously-adsorbed dye from theirsurfaces by mixing them with a magnetic photocatalyst (typicallyanatase-TiO₂-coated (cycle-5) SiO₂/γ-Fe₂O₃) in an aqueous solution andexposing the resulting suspension to the solar-radiation. It is obviousthat the other “core-shell” type magnetic photocatalysts havingdifferent magnetic core such as CoFe₂O₄, NiFe₂O₄, MnFe₂O₄, Ni, Fe, Co,and others, in combination with the shell of other nanocrystallinesemiconductor photocatalyst such as ZnO, CdS, ZnS, and others, are alsosuitable for recycling the as-received flyash particles and themicro-nano integrated HTN-flyash composite particles for the dye-removalapplication. The magnetic photocatalyst having the morphology other thanthe “core-shell” structure, such as the nanoparticles of magneticmaterial deposited on the surface of semiconductor-oxide nanotubes maybe also used.

Example-7

In this example, the values of different parameters used for theprocessing of micro-nano integrated HTN-flyash composite particles areidentical with those already described in the Example 1 except for thefollowing changes. The SnCl₂ precursor having the initial concentrationof 40 g·l⁻¹ is replaced with Fe(NO₃)₃.9H₂O precursor having the initialconcentration of 20 g·l⁻¹, for surface-sensitizing the as-receivedflyash particles with Fe³⁺ cations instead of Sn²⁺. By this process, theHTN-flyash composite particles (non-magnetic) containing 20 wt. % Fe and7 wt. % HTN (balance being the weight of flyash particles) are obtained.

The TEM images and the EDX pattern as obtained using the HTN-flyashcomposite particles, with the surface-sensitization of flyash particlesobtained using the Fe³⁺ cations, are presented in FIGS. 16(a)-(b) and16(c). Similar to the observation made in FIG. 4(b), the HTN are notedto be attached to or anchored to the surface of flyash particles,surface-sensitized with the Fe³⁺ cations, typically at the short-edges(tube-openings) which confirms the operation of ion-exchange mechanismoperating under the dark-condition in an aqueous solution. Thecomparison of EDX spectrum with that of as-received flyash particles,FIG. 3(a), show an increase in the intensity of Fe and Ti peaks,suggesting the formation of HTN-flyash composite particles via the Fe³⁺sensitization.

The as-received flyash particles and the HTN-flyash composite particlesare then utilized in the MB dye-adsorption experiments, conducted in thedark-condition, as described earlier in the Example-2 except that theinitial MB dye concentration of 30 μM is used in this example. Thevariation in the normalized concentration of surface-adsorbed MB dye asa function of contact time, as obtained for the as-received flyashparticles and micro-nano integrated HTN-flyash composite particles(latter surface-sensitized with Fe³⁺ ions), is presented and compared inFIG. 17. Relative to the equilibrium MB dye adsorption exhibited by theas-received flyash particles, the amount of MB dye adsorbed atequilibrium is seen to enhance due to the anchoring of HTN on thesurface of flyash particles which are pre-sensitized with the Fe³⁺cations.

Example-8

In this example, the values of different parameters used for theprocessing of micro-nano integrated HTN-flyash composite particles areidentical with those already described in the Example 1 except for thefollowing changes. The SnCl₂ precursor is not utilized forsurface-sensitizing the as-received flyash particles with Sn²⁺ cations.The surface-sensitization of as-received flyash particles is achievedusing the adsorption of H⁺ ions instead. The contact time of HCIsolution with the as-received flyash particles is 1 h instead of 4 h. Bythis process, the HTN-flyash composite particles (non-magnetic)containing ˜2 wt. % H and 9 wt. % HTN (balance being the weight offlyash particles) are obtained.

The TEM image and EDX pattern of HTN-flyash composite particles,processed via the surface-sensitization of flyash using the H⁺ ions, arepresented in FIGS. 18(a) and 18(b). The as-received flyash particles andthe HTN-flyash composite particles are then utilized in the MBdye-adsorption experiments, conducted in the dark-condition, asdescribed earlier in the Example-2 except that the initial MB dyeconcentration of 15 μM is used in this example. The variation in thenormalized concentration of surface-adsorbed MB dye as a function ofcontact time, as obtained for the as-received flyash particles andmicro-nano integrated HTN-flyash composite particles (with lattersurface-sensitized with H⁺ ions), is presented and compared in FIG. 19.Relative to the equilibrium MB dye adsorption exhibited by theas-received flyash particles, the amount of MB dye adsorbed atequilibrium is seen to enhance due to the anchoring of HTN on thesurface of flyash particles which are pre-sensitized with the H⁺ ions.

Example-9

In this example, the HTN-γ-Fe₂O₃ magnetic nanocomposite is synthesizedvia an innovative approach involving an ion-exchange mechanism operatingunder the dark-condition in an aqueous solution. As mentioned in theExample 7, Fe³⁺ cations can be used as the surface-sensitizer for theas-received flyash particles to anchor the HTN to their surfaces. Sincethe magnetic γ-Fe₂O₃ is the oxide of Fe³⁺ ions, thesurface-sensitization step is not necessary and may be eliminated toanchor the HTN to the surface of γ-Fe₂O₃ via an ion-exchange mechanismoperating under the dark-condition in an aqueous solution.

To demonstrate this, 0.5 g (or 0.1 g) of γ-Fe₂O₃ are dispersed in 125 mlof distilled-water having the neutral solution-pH (˜6.5-7.5) under thecontinuous overhead stirring at 25° C. for 10 min. 0.5 g (or 0.9 g) ofhydrothermally processed HTN are then added to the above suspensionunder the continuous overhead stirring at 25° C. for 10 min. Theresulting suspension is sonicated for 10 min and then stirredcontinuously under the dark-condition at 25° C. for 8 h using anoverhead stirrer. The nano-nano integrated HTN-γ-Fe₂O₃ magneticnanocomposite particles, thus formed, are separated from the aqueoussolution using an external magnetic field, washed using 100 ml ofdistilled water for 1 h multiple-times till the pH of filtrate remainsunchanged or neutral, again separated from the aqueous solution using anexternal magnetic field, and then dried in an oven at 80° C. for 12 h toobtain the HTN-γ-Fe₂O₃ magnetic nanocomposite particles containing 50wt. % (or 10 wt. %) of γ-Fe₂O₃ particles (balance being the weight ofHTN).

The TEM image of as-received nanocrystalline γ-Fe₂O₃ particles is shownin FIG. 20(a) and the corresponding EDX spectrum is presented in FIG.20(b). The size of as-received nanocrystalline γ-Fe₂O₃ magneticparticles is within the range of 15-25 nm having near-sphericalmorphology. The EDX spectrum confirms the presence Fe and O within thesenanoparticles. The selected-area electron diffraction (SAED) pattern;shown as an inset in FIG. 20(a), exhibits the concentric ring patternindicating the nanocrystalline nature of as-received γ-Fe₂O₃nanoparticles in agreement with the TEM image. The XRD pattern ofas-received γ-Fe₂O₃ nanoparticles is presented in FIG. 20(c). Theas-received magnetic nanoparticles possesses maghemite (γ-Fe₂O₃)structure as confirmed after comparing the XRD pattern with the JCDPScard no. 39-1346. The TEM images of HTN-γ-Fe₂O₃ magnetic nanocompositesare shown in FIG. 21, which clearly shows the attaching or anchoring ofHTN at the short-edges (tube-openings) to the surface of γ-Fe₂O₃magnetic nanoparticles via an ion-exchange mechanism operating under thedark-condition in an aqueous solution. It is noted that the magneticnanoparticles are not dispersed along the surface of HTN which makeslarge number of potential-sites available for the dye-adsorptionprocess, which could not be possible if the magnetic nanoparticles wouldhave been dispersed along the surface of HTN. It appears that theattaching or anchoring of HTN typically at the short-edges(tube-openings) to the surface of γ-Fe₂O₃ magnetic nanoparticles via anion-exchange mechanism takes place only at the neutral solution-pH.Similar process conducted in an acidic or basic range may lead to theformation of HTN-γ-Fe₂O₃ magnetic nanocomposites with the magneticnanoparticles dispersed along the surface of HTN via the electrostaticattraction mechanism involving both the coulombic and van der Waalsforces. The EDX pattern obtained using the HTN-γ-Fe₂O₃ magneticnanocomposites, corresponding to FIG. 21(b), is shown in FIG. 22 whichshows the presence of Ti and Fe as the major elements confirming theformation of nanocomposite particles. It is to be noted that the γ-Fe₂O₃magnetic nanoparticles are not surface-sensitized for attaching oranchoring the HTN to their surfaces, which is however an essential stepin the case of as-received flyash particles as described in theExample 1. In FIG. 23, the use of HTN-γ-Fe₂O₃ magnetic nanocompositeparticles for the removal of MB dye from an aqueous solution, via thesurface-adsorption process involving the ion-exchange andelectrostatic-attraction mechanisms operating in the dark-condition hasbeen demonstrated. The separation of HTN-γ-Fe₂O₃ magnetic nanocompositeparticles, after the MB dye adsorption on the surface, using a moderateexternal magnetic field is clearly visible.

Thus, the processing of semiconductor-oxide nanotubes (HTN)-metal oxide(γ-Fe₂O₃) magnetic nanocomposite particles is successfully demonstrated,without the surface-sensitization step, via an innovative processinvolving an ion-exchange mechanism operating under the dark-conditionin an aqueous solution.

Example-10

Similar to the Example 6, the previously adsorbed-dye can be decomposedon the surface of HTN-γ-Fe₂O₃ magnetic nanocomposite by mechanicallymixing and suspending it with the non-magnetic nanocrystallineanatase-TiO₂ photocatalyst (˜3-30 nm average diameter) in an aqueoussolution and exposing the resulting suspension to the UV orsolar-radiation under the continuous overhead stirring till theadsorbed-dye is completely decomposed. Large concentration of OH. isproduced by the nanocrystalline anatase-TiO₂ photocatalyst in shortertime relative to that produced by the HTN and γ-Fe₂O₃, FIG. 24, whichattack and decompose the MB dye adsorbed on the surface of magneticnanocomposite particles in relatively less time. The HTN-γ-Fe₂O₃ (5 wt.%) magnetic nanocomposite particles (1.0 g) are first prepared using theprocedure described in the Example-9 except that the amount of γ-Fe₂O₃and HTN used are changed to 0.05 g and 0.95 g. The magneticnanocomposite particles are then suspended and stirred at 25° C. for 1 hin the dark-condition using an overhead stirrer in 125 ml aqueoussolution of MB dye (30 μM). After the magnetic separation, the magneticnanocomposite particles with the surface-adsorbed MB dye are dried in anoven at 80° C. for 12 h. Two different sets of samples are prepared. Oneset of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles with thesurface-adsorbed MB dye are then used for the dye-adsorption experimentconducted with the initial MB dye concentration of 100 μM. The secondset of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles with thesurface-adsorbed MB dye are subjected to the surface-cleaning treatment.In this treatment, 1.0 g of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocompositeparticles with the surface-adsorbed MB dye (1.2 mg·g⁻¹) is suspended in125 ml aqueous solution under the continuous overhead stirring. 1.0 g(50 wt. % of total amount of suspended solid particles) of non-magneticnanocrystalline anatase-TiO₂ photocatalyst is then added. The resultingsuspension is sonicated for 15 min and then stirred at 25° C. using anoverhead stirrer under the UV-radiation exposure for 2 h. The mixture ofsurface-cleaned magnetic nanocomposite particles and the non-magneticnanocrystalline anatase-TiO₂ photocatalyst are separated together usinga centrifuge operated at 3000 rpm and washed using 100 ml ofdistilled-water for 1 h multiple-times till the pH of filtrate remainsunchanged or neutral. The surface-cleaned magnetic nanocompositeparticles are separated from the non-magnetic nanocrystallineanatase-TiO₂ photocatalyst using an external magnetic field. Theseparated powders are dried in an oven at 80° C. for 12 h. Thesurface-cleaned magnetic nanocomposite particles are recycled for thenext-cycle of dye-adsorption conducted in the dark-condition using anaqueous solution containing 100 μM of MB dye.

The variation in the normalized concentration of surface-adsorbed MB dyeas a function of contact time as obtained for the HTN-γ-Fe₂O₃ (5 wt. %)magnetic nanocomposite particles, before and after surface-cleaningtreatment, is presented in FIG. 25. In 30 μM MB dye solution, theHTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles adsorb almost100% of dye on the surface. If the MB dye adsorption is conducted,without the surface-cleaning treatment, using an aqueous solutioncontaining 100 μM of MB dye, then the amount of dye-adsorbed on thesurface of nanocomposite particles is noted be 50%. On the other hand,if the MB dye adsorption is conducted, after the surface-cleaningtreatment, using an aqueous solution containing 100 μM of MB dye, thenthe amount of dye-adsorbed on the surface of nanocomposite particles isnoted be 90%. The increase in the amount of dye-adsorption after thesurface-cleaning treatment is, thus, clearly visible. This stronglysuggests the successful decomposition of previously adsorbed-dye on thesurface of HTN-γ-Fe₂O₃ (5 wt. %) magnetic nanocomposite particles bymixing them with the non-magnetic nanocrystalline anatase-TiO₂photocatalyst in an aqueous solution and subjecting the suspension tothe UV-radiation. It is obvious that any other non-magneticphotocatalyst, including CdS, ZnS, and other, may be selected for thesurface-cleaning treatment instead of anatase-TiO₂.

It is to be noted that the pure nanocrystalline anatase-TiO₂ used forthe surface-cleaning treatment get attached to (or anchored to) theHTN-γ-Fe₂O₃ magnetic nanocomposite particles via an ion-exchangemechanism operating the under the dark-condition in an aqueous solution.As a result, some amount of (non-magnetic) photocatalyst particles islost after the magnetic separation. This has been observed to affect theamount of dye-adsorbed by the HTN-γ-Fe₂O₃ magnetic nanocompositeparticles in the subsequent dye-adsorption cycles (2^(nd) cycle)following the surface-cleaning treatment. The latter issue becomessevere with the increasing number of dye-adsorption cycles. This is alsoparticularly noted for the industry effluent sample containing a coldreactive dye (Red M5B). In order to avoid the interaction between theHTN-γ-Fe₂O₃ magnetic nanocomposite particles and the nanocrystallineanatase-TiO₂ photocatalyst particles, (that is to minimize the operationof an ion-exchange mechanism in between the two), the latter is requiredto be coated with the noble-metal selected from the group of Pt, Au, Pd,Ag, nanoparticles which can not only reduce the said interaction butalso help enhance the concentration of OH. produced under theradiation-exposure. This has been demonstrated in the following exampleusing the above industry effluent sample received from the RubmachIndustries, Ahmedabad, Gujarat, India.

Example-11

In this example, Pt is first deposited on pure anatase-TiO₂ via theUV-reduction process. First, platinum(II) chloride (PtCl₂) is dissolvedin a proper concentration range in 200 ml of distilled H₂O under thecontinuous magnetic stirring. 1 g of pure anatase-TiO₂ is then dispersedso as to obtain the Pt/Ti mole-ratio varying in the range of 1, 5, and10%. The initial solution-pH is then adjusted to ˜10 by the addition ofNH₄OH solution. The resulting suspension is then exposed to theUV-radiation in a Photoreactor (Luzchem, Canada) for 4 h. ThePt-deposited nanocrystalline anatase-TiO₂ is separated using acentrifuge operated at 3000 rpm and dried in an oven at 80° C. for 12 h.A typical TEM image of Pt-deposited nanocrystalline anatase-TiO₂ havingthe Pt/Ti mole-ratio of 5% is shown in FIG. 26(a) where very fine Ptnanoparticles dispersed on the surface of one of the nanocrystallineanatase-TiO₂ particle is clearly seen.

In order to prepare HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocompositeparticles, the procedure already described in the Example-9 is followed.0.3 g of HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles arefirst suspended and stirred using an overhead stirrer in thedark-condition at 25° C. for 1 h in 125 ml aqueous solution containing 1vol. % industry effluent sample. (NOTE: The industry effluent samplecontains 10% of cold reactive Red M5B dye). The magnetic nanocompositeparticles with the surface-adsorbed reactive-dye are separated using amagnetic separator and dried in an oven at 80° C. for 12 h. Thesecond-cycle of dye-adsorption is conducted using the dried magneticnanocomposite particles having the previously adsorbed reactive-dye onthe surface. In another set of experiments, HTN-γ-Fe₂O₃ (10 wt. %)magnetic nanocomposite particles with the surface-adsorbed reactive-dyeare prepared, which are subjected to the surface-cleaning treatmentunder the UV-radiation exposure using the Pt-deposited nanocrystallineanatase-TiO₂ having the varying Pt/Ti mole-ratio within the range of1-10%. During the surface-cleaning treatment, 0.3 g of HTN-γ-Fe₂O₃ (10wt. %) magnetic nanocomposite particles with the surface-adsorbedreactive-dye is suspended in 100 ml aqueous solution under thecontinuous overhead stirring at 25° C. for 10 min. 0.3 g (50 wt. % oftotal amount of suspended solid particles) of non-magnetic Pt-depositednanocrystalline anatase-TiO₂ photocatalyst is then added under thecontinuous overhead stirring at 25° C. for 10 min. The resultingsuspension is sonicated for 10 min and then stirred using an overheadstirrer under the UV-radiation exposure for 5 h. Due to thePt-deposition, the HTN within the magnetic nanocomposite particles arenot attached to (or anchored to) the nanocrystalline anatase-TiO₂photocatalyst particles. The mixture of surface-cleaned magneticnanocomposite particles and the non-magnetic nanocrystallinePt-deposited anatase-TiO₂ photocatalyst are separated together using acentrifuge operated at 3000 rpm and washed using 100 ml ofdistilled-water for 1 h multiple-times till the pH of filtrate remainsunchanged or neutral. The surface-cleaned magnetic nanocompositeparticles are separated from the non-magnetic Pt-depositednanocrystalline anatase-TiO₂ photocatalyst using an external magneticfield. The separated powders are dried in an oven at 80° C. for 12 h.The surface-cleaned magnetic nanocomposite particles are then recycledfor the next-cycle of dye-adsorption conducted in the dark-condition.The amount of reactive-dye remaining in the solution, after eachdye-adsorption experiments, is monitored via the UV-visible absorptionspectrophotometer.

The variation in the UV-visible absorption intensity obtained after thedye-adsorption experiments conducted under the different conditions, asobserved for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocompositeparticles, is presented in FIG. 26(b). The cold reactive dye Red M5Bexhibits the characteristic absorbance peak located at ˜500 nm. Thevariation in the intensity of this peak is monitored to judge therelative concentration of dye remaining in the aqueous solution afterthe dye-adsorption experiments. The intensity of absorbance-peak ishigher for the initial dye-solution. The intensity however reducessignificantly for the HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocompositeparticles during the 1^(st) cycle of dye-adsorption. Nevertheless, whenthe HTN-γ-Fe₂O₃ (10 wt. %) magnetic nanocomposite particles with thepreviously adsorbed-dye is used for the 2^(nd) cycle of dye-adsorption,no significant dye-adsorption on the surface is noted. This stronglysuggests that after the 1^(st) cycle of dye-adsorption, asurface-cleaning treatment is required to decompose the previouslyadsorbed-dye. Hence, the surface-cleaning treatment is conducted usingthe combination of Pt-deposited nanocrystalline anatase-TiO₂ having thePt/Ti mole-ratio varying in the range of 1-10% and the UV-radiationexposure. It is observed that the dye-adsorption in the 2^(nd) cycle isenhanced considerably following the surface-cleaning treatment. It isfurther noted that the Pt-deposited nanocrystalline anatase-TiO₂ havingthe Pt/Ti mole-ratio of 10% exhibits the best performance during thesurface-cleaning treatment. This has been attributed to both thesignificant reduction in the interaction between the HTN-γ-Fe₂O₃ (10 wt.%) magnetic nanocomposite particles and the nanocrystalline anatase-TiO₂photocatalyst particles (that is, the minimization of operation of anion-exchange mechanism in between the two) and enhanced concentration ofOH. produced under the UV-radiation exposure due to the photo-inducedelectron-trapping effect of Pt. The pure nanocrystalline anatase-TiO₂ isnot effective for the surface-cleaning treatment and increasing thedye-adsorption during the 2^(nd) cycle; hence, it is not included inFIG. 26(b) for clarity. It is obvious that the other noble-metals suchas Au, Pd, Ag, and others may also work instead of Pt.

Example-12

In this example, the values of different parameters used for theprocessing of HTN-γ-Fe₂O₃-flyash magnetic composite particles areidentical with those already described in the Example 1 except for thefollowing changes. The initial SnCl₂ concentration is changed to 20g·l⁻¹. The pure-HTN is replaced with HTN-γ-Fe₂O₃ (10 wt. %) magneticnanocomposite which are processed using the method as already describedin the Example 9. Hence, the final HTN-γ-Fe₂O₃-flyash magnetic compositeparticles contain ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being theweight of Sn²⁺-sensitized flyash particles).

The TEM image and EDX spectrum obtained using the magneticHTN-γ-Fe₂O₃-flyash composite particles, containing ˜1 wt. % γ-Fe₂O₃ and˜8 wt. % HTN (balance being the weight of Sn²⁺-sensitized flyashparticles), are presented in FIGS. 27(a) and 27(b). The comparison ofFIG. 27(a) with FIG. 4(b) suggests the additional presence of magneticγ-Fe₂O₃ nanoparticles on the surface of flyash particles which arepre-anchored to the HTN via the ion-exchange process operating under thedark-condition in an aqueous solution. This is further supported by theEDX analysis which shows (after the comparison with FIG. 3(a)) anincrease in the relative intensities of Ti and Fe on the surface ofas-received flyash particles. The magnetic γ-Fe₂O₃ nanoparticles providethe magnetic property; while, the HTN provide an enhanced dye-adsorptioncapacity to the composite particle. Thus, the processing of flyash-basedmagnetic dye-adsorbent composite is successfully demonstrated in thisexample.

The effect of vertically held magnet (M) on the as-received flyashparticles and magnetic HTN-γ-Fe₂O₃-flyash composite particles,containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weightof Sn²⁺-sensitized flyash particles), is compared in FIG. 28. Asindicated by the arrows, the magnetic HTN-γ-Fe₂O₃-flyash compositeparticles strongly adhere to the vertically held magnet, FIG. 28(b),relative to that exhibited by the as-received flyash particles, FIG.28(a). This strongly suggests that the HTN-γ-Fe₂O₃-flyash compositeparticles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance beingthe weight of Sn²⁺-sensitized flyash particles), can be separated froman aqueous solution using an external magnetic field which Is useful inthe dye-removal application. Very weak magnetic property is alsopossessed by the as-received flyash particles possibly due to thepresence of small amount of Fe₂O₃ in their structure, which is howevernot strong enough for their separation from an aqueous solution using anexternal magnetic field.

The as-received flyash particles and the HTN-γ-Fe₂O₃-flyash magneticcomposite particles, containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN(balance being the weight of Sn²⁺-sensitized flyash particles), are thenutilized in the MB dye-adsorption experiments, conducted in thedark-condition, as described earlier in the Example-2 except that theinitial MB dye concentration of 7.5 μM is used in this example. Thevariation in the normalized concentration of surface-adsorbed MB dye asa function of contact time, as obtained for the as-received flyashparticles and HTN-γ-Fe₂O₃-flyash magnetic composite particles,containing ˜1 wt. % γ-Fe₂O₃ and ˜8 wt. % HTN (balance being the weightof Sn²⁺-sensitized flyash particles), is presented and compared in FIG.29(a). Relative to the equilibrium MB dye adsorption exhibited by theas-received flyash particles, the amount of MB dye adsorbed atequilibrium is seen to be enhanced due to the anchoring of HTN-γ-Fe₂O₃nanocomposite to the surface of flyash particles which arepre-sensitized with the Sn²⁺ cations. After the dye-adsorption process,the HTN-γ-Fe₂O₃-flyash magnetic composite particles could be separatedfrom the treated aqueous solution using an external magnetic field, FIG.29(b).

Thus, the processing of magnetic flyash-based composite particles,having an enhanced dye-adsorption capacity with the ability of magneticseparation, via an innovative approach is successfully demonstrated inthis example. It is obvious that the magnetic flyash-based compositeparticles can be recycled in the dye-removal application using thetechnique similar to that used for the recycling of semiconductor-oxidenanotubes-metal oxide magnetic nanocomposite particles as demonstratedin the Example-11.

Example-13

In this example, the HTN-SnO composite particles are synthesized via aninnovative approach involving an ion-exchange mechanism operating underthe dark-condition in an aqueous solution. As demonstrated in theExample 1, Sn²⁺ cations can be used as the surface-sensitizer for theas-received flyash particles to anchor the HTN to their surfaces. SinceSnO is the oxide of Sn²⁺ cations, the surface-sensitization step is notnecessary and may be eliminated to anchor the HTN to the surface of SnOvia an ion-exchange mechanism operating under the dark-condition in anaqueous solution.

To demonstrate this, 0.9 g of SnO particles (Sigma-Aldrich Chemicals,Bangalore, India) are dispersed in 100 nil of distilled-water under thecontinuous overhead stirring at 25° C. for 10 min. 0.1 g ofhydrothermally processed HTN are then added to the above suspensionunder the continuous overhead stirring at 25° C. for 10 min. Theresulting suspension is sonicated for 10 min and then stirredcontinuously under the dark-condition for 1 h using an overhead stirrer.The nano-nano integrated (10 wt. %) HTN-SnO composite particles, thusformed, are separated from the aqueous solution using a centrifugeoperated at 3000 rpm, washed using 100 ml of distilled-water for 1 hmultiple-times till the pH of filtrate remains unchanged or neutral,again separated from the aqueous solution via centrifuging, and thendried in an oven at 80° C. for 12 h to obtain the (10 wt. %) HTN-SnOcomposite particles.

The TEM image and EDX analysis of the (10 wt. %) HTN-SnO compositeparticles, processed via ion-exchange mechanism operating under thedark-condition in an aqueous solution, are presented in FIGS. 30(a) and30(b) respectively. The corresponding SAED pattern is shown as an insetin FIG. 30(a). The anchoring of HTN at the shorter tube-edges(tube-openings) to the surface of SnO particle is clearly visible. Thus,the formation of (10 wt. %) HTN-SnO composite particles processed via anion-exchange mechanism, operating under the dark-condition in an aqueoussolution, without the involvement of surface-sensitization step issuccessfully demonstrated.

ADVANTAGES OF THE INVENTION

-   1. It provides the innovative semiconductor-oxide nanotubes-based    composite products (both magnetic and non-magnetic).-   2. It provides the innovative methods for the processing of    semiconductor-oxide nanotubes-based composite products (both    magnetic and non-magnetic).-   3. It provides a new industrial application to the innovative    semiconductor-oxide nanotubes-based composite products (both    magnetic and non-magnetic), typically the industrial dye-removal    from the aqueous solutions.-   4. It provides an innovative approach to increase the specific    surface-area of flyash without affecting its spherical morphology.-   5. It provides an innovative method to recycle the    semiconductor-oxide nanotubes-based products (both magnetic and    non-magnetic) used in the industrial dye-removal application.-   6. It provides an innovative solution for tackling the handling,    disposal, and recycling issues associated with the flyash without    and with the surface-adsorbed metal-cations.

We claim:
 1. A composite particle which comprises: (i) metal-cations(M^(n+)) or protons-sensitized flyash particle of diameter in the rangeof 0.3-100 μm, wherein the flyash particle consists of a mixture of50-85 wt % silica, 5-20 wt % alumina, 5-15 wt % iron oxide, at least oneselected from the group consisting of calcium oxide, titanium oxide andmagnesium oxide, and at least one toxic heavy metal selected from thegroup consisting of arsenic, lead and cobalt; and wherein themetal-cations (M^(n+)) are selected from the group consisting ofSn²⁺/Sn⁴⁺, Fe²⁺/Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, and Ti⁴⁺; and (ii)nanotubes of semiconductor-oxides deposited on the surface ofmetal-cations or protons-sensitized flyash particle, wherein thenanotubes of semiconductor-oxides are selected from the group consistingof hydrothermally processed hydrogen titanate (HTN, H₂Ti₃O₇ or thelepidocrocite-type), anatase-TiO₂ (ATN), and Ag-doped ATN.
 2. Thecomposite particle as claimed in claim 1, wherein the nanotubes are inthe range of 1-15 wt. %, the metal-cations (M^(n+)) or protons in therange of 1-80 wt. %, and the balance being the weight of flyashparticles.
 3. The composite particle as claimed in claim 1, wherein thenanotubes of semiconductor-oxides are attached to the metal-cations(M^(n+)) or protons-sensitized flyash particle at the short-edges(tube-openings) and/or surface-deposited with the metal-cations (M^(n+))or protons-sensitized flyash particle.
 4. The composite particle asclaimed in claim 1 is useful for the application involving thedye-removal from an aqueous solution and industry-effluent via thesurface-adsorption mechanism operating in the dark-condition.
 5. Aprocess for the preparation of the composite particle as claimed inclaim 1, wherein said process comprises: (a) dissolving a metal-salt(1-70 g·l⁻¹) in water having an initial solution-pH of about 1-2adjusted using the 0.1-1 M HCI solution under continuous stirring for30-300 min at temperature ranging between 20-30° C., (b) suspendingflyash particles (1-25 g·l⁻¹) into it under continuous stirring for30-300 min at temperature ranging between 20−30° C. to obtain thesuspension of metal-cations (M^(n+)) sensitized flyash particles; (c)optionally, suspending flyash particles (1-25 g·l⁻¹) in 50-250 ml ofwater having the initial solution-pH of about 1-2 adjusted using the0.1-1 M HCI solution under continuous stirring for 30-300 min attemperature ranging between 20-30° C. to obtain the suspension ofprotons-sensitized flyash particles; (d) adding hydrothermally processednanotubes of semiconductor-oxides (0.1-5 g·l⁻¹) to the suspension ofcations (M^(n+)) or protons-sensitized flyash particles as obtained instep (b) or in step (c), followed by sonicating the suspension for 5-30min; (e) stirring the suspension obtained in step (d) continuously for30-300 min at temperature ranging between 20−30° C. to obtain thecomposite of semiconductor-oxide nanotubes and cations (M^(n+)) orprotons-sensitized flyash particles; and (f) separating the compositeparticles as obtained in step (e) using a centrifuge operated at2000-4000 rpm or an external magnetic field, followed by washing thecomposite particles using water for 30 min-2 h multiple-times until thepH of filtrate remains unchanged or neutral, followed by separation anddrying the composite particles in an oven at 70-90° C. for 10-15 h. 6.The process as claimed in claim 5, wherein the metal-salt used in step(a) is selected from the group consisting of chloride, nitrate,sulfate-salts of Sn⁺/Sn⁺, Fe²7Fe³⁺, Pb²⁺, Zn²⁺, Cu²⁺, Mn²⁺, Ti⁺, andothers.
 7. A process for the surface-cleaning and the recycling ofcomposite/nanocomposite particles as claimed in claim 1 after theadsorption of an organic synthetic-dye from an aqueous solution via thesurface-adsorption mechanism operating in the dark-condition,comprising: a. suspending 1-30 g·l⁻¹ of composite/nanocompositeparticles having the surface-adsorbed organic synthetic-dye (0.1-3mg·g⁻¹) in water under continuous stirring for a period ranging between5-30 min at temperature ranging between 20-30° C.; b. suspending aphotocatalyst (10-60 wt. % of total weight of suspended solid particles)selected from the group of nanocrystalline anatase-TiO₂-coatedSiO₂/γ-Fe₂O₃ magnetic photocatalyst or the noble-metal-depositednanocrystalline anatase-TiO₂ in the suspension obtained in step (a)under continuous stirring, followed by sonicating the suspension for5-30 min, subsequently stirring the suspension continuously under the UVor solar-radiation exposure for 1-10 h; c. centrifuging the solution at2000-4000 rpm to separate the composite/nanocomposite particles andphotocatalyst together, followed by washing using water for 30 min-2 hmultiple-times until the pH of filtrate remains unchanged or neutral; d.separating the photocatalyst particles from the surface-cleanedcomposite/nanocomposite particles using an external magnetic field; ande. drying both the photocatalyst particles and composite/nanocompositeparticles in an oven at 70-90° C. for 10-15 h for the reuse.